A simple little word for a common little fella. Yet, the origin is unknown (or not?).

Both OED and Etymonline are bold enough to say "of uncertain origin"; but, of course, they try to explain the etymology of rat with Latin, Romanic, Germanic, Celtic and Greek connections.

Etymonline summarizes in the first paragraph as follows:

late Old English ræt "rat," of uncertain origin. Similar words are found in Celtic (Gaelic radan), Romanic (Italian ratto, Spanish rata, French rat) and Germanic (Old Saxon ratta; Dutch rat; German Ratte, dialectal Ratz; Swedish råtta, Danish rotte) languages, but connection is uncertain and origin unknown. In all this it is very much like cat.

(a hint for the next question: cat!)

Etymonline gives its own opinion also and includes the dispute between big names:

  • Perhaps from Vulgar Latin * rattus
  • Weekley thinks this is of Germanic origin, "the animal having come from the East with the race-migrations" and the word passing thence to the Romanic languages
  • American Heritage and Tucker connect Old English ræt to Latin rodere and thus PIE * red- "to scrape, scratch, gnaw," source of rodent (q.v.).
  • Klein says there is no such connection and suggests a possible cognate in Greek rhine "file, rasp."
  • Weekley connects them with a question mark and Barnhart writes, "the relationship to each other of the Germanic, Romance, and Celtic words for rat is uncertain.
  • OED says "probable" the rat word spread from Germanic to Romanic, but takes no position on ultimate origin.

The connection to the origin of rodent (Latin rodere "to gnaw, eat away," from PIE root red-) stands out but not favored by Klein and OED and it is mentioned as an uncertain connection.
(Related question: What do rodents do?)

OED goes deeper into the rathole and gives a detailed etymology; and includes the following reasons for the uncertainty:

  • It is uncertain whether the Latin and Romance words are cognate with the Germanic words, or whether they were borrowed from Germanic, or vice versa; in any case the ultimate origin is uncertain; perhaps imitative of the sound of gnawing.
  • None of the Latin and Romance words is attested before the end of the first millennium, and the fact that the German word has not undergone the High German sound shift suggests that the Germanic group is also late (Middle High German ratz , ratze , German regional (chiefly southern) Ratz , Ratze are secondary, perhaps hypocoristic formations).
  • The word was probably spread with the reintroduction of rats to Northern Europe during the Viking Age (for a discussion of the physical evidence compare P. L. Armitage in Antiquity 68 (1994) 231–40).
  • A derivation < an ablaut variant of the Indo-European base of classical Latin rōdere to gnaw (see rodent adj.) has been suggested, but seems unlikely in the light of the apparently recent introduction of the word.
  • A suggested derivation of the Romance words < classical Latin rapidus rapid adj. is no longer accepted, as it would only account for the Italian, which for chronological and historical reasons cannot be the single origin of the whole group.

But, I didn't stop ratting there and checked historical records that include the word "rat" and found the oldest etymological and lexical references as follows:

From "An universal etymological English dictionary" By Nathan Bailey (1731):

a RAT [rat, F. ratta, Span. ratze, Teut. ratte, Du.] an amphibious kind of Animal, infesting Houses, Ships, &c.

[(obsolete) Teut. -> Teutonic -> Germanic]

From "A dictionary of the English language : in which the words are deduced from their originals, and illustrated in their different significations by examples from the best writers : to which are prefixed, a history of the language, and an English grammar" By Samuel Johnson (1766):

RAT. f. [ratte, Dutch; rat, French; ratta, Spanish.] An animal of the mouse kind that infests houses and ships

And the rat-hunting went on an' on and I ended up with the brown rat (and its etymological origin) which is one of the best known and most common rat. It is named Rattus norvegicus (Norwegian rat) which is a misnomer because the English naturalist John Berkenhout gave the binomial name, believing it had migrated to England from Norwegian ships in 1728 (which is disproven later.)

Towards the end of 19th century, the etymology of brown rat was better understood and the following note is mentioned in "Natural History" by American scholar Alfred Henry Miles:

The brown rat is the species common in England, and best known throughout the world. It is said to have travelled from Persia to England less than two hundred years ago and to have spread from thence to other countries visited by English ships

This finding gives a hint that the origin of the word rat might be of Persian origin (and also Sanskrit) [Wiktionary puts as "Middle Persian randītan (“to scrape, smooth”), Sanskrit rádati (“he gnaws, cuts”))."]

Also mentioned in "The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World" By J. P. Mallory, D. Q. Adams:

The Engligh rat takes its name from *red- 'gnaw, scrape' (cf. also Lat rōdō 'gnaw', MPers randītan 'scrape, smooth', Skt rádati 'bites, gnaws, cuts, makes way, opens').

However, the brown rat ultimately came from central Asia and (likely) China. The pinyin shǔ of Han character 鼠 (rat) in Mandarin doesn't evoke anything, though I'm like a rat in a corner at this point. [Although, Sanskrit and Mandarin are from the same geographical region.)

In conclusion, Persian and Sanskrit origin makes sense and it wouldn't be possible to go further back (unless a Hittite origin? Rats!).

Anything more to add? Is it possible to find further clarification?

Sorry, I couldn't think of a fancy title for the question.

  • 1
    Well the Japanese certainly don't say rat, so it can't have come from there. They say nezumi and have no separate word for mouse. There are big nezumi and small nezumi.
    – WS2
    Feb 19, 2015 at 8:13
  • @WS2 that's certainly interesting but do you know anything about when rats reached Japan? After all it's an island nation that has at various times in its history chosen to cut itself off to some extent from other countries and therefore their shipping.
    – Chris H
    Feb 19, 2015 at 9:05
  • @ChrisH I have no idea. Perhaps from China, whose traders took to the Japanese the influence of their language and writing system - perhaps rats came as part of the package!
    – WS2
    Feb 19, 2015 at 9:26
  • @WS2 perhaps. I ask because if "mouse" and "rat" share a word in Japanese, and mice were common before rats arrived, calling the new pest a "big mouse" would be sensible. It wouldn't have much benefit to linguists though. This pattern of naming a newly-encountered species after a familiar one is perhaps more common in the colonisation of the new world -- "elk", "sparrow" etc.
    – Chris H
    Feb 19, 2015 at 10:54
  • 1
    @MarcInManhattan Typos, sorry (the pictures are still linked just in case).
    – Laurel
    Dec 8, 2021 at 15:25

4 Answers 4


In Old Norse, Ratatoskr means "drill-tooth" or "bore-tooth". It is the name of a mythical creature, a squirrel that runs up and down the tree of life called Yggdrasil, acting as a messenger between two arch enemies: the great eagle and the terrestrial dragon.

According to Albert Sturtevant, "[as] far as the element Rata- is concerned, Bugge's hypothesis has no valid foundation in view of the fact that the [Old Norse] word Rata (gen. form of Rati*) is used in Háv[amál] to signify the instrument which Odin employed for boring his way through the rocks in quest of the poet's mead [...]" and that "Rati* must then be considered a native [Old Norse] word meaning "The Borer, Gnawer"


One can therefore assume that the word rat stems from a description of its behaviour.

  • Do you have a reference for that Old Norse definition, and any evidence that it also applies to English? Sep 12, 2015 at 13:33

On the Isle of Man a rat was known by the term 'queer fellow' , a very old saying and one l don't know whether is still in use. It was often used alongside the word 'frickened'.

  • 3
    Your name is strangely appropriate for this answer. Apr 24, 2015 at 14:51
  • 1
    @JATerroba "There is no shortage of superstitions on the Isle of Man. One of them is that if you are Manx, you do not use the word - RAT" transceltic.com
    – DjinTonic
    Dec 8, 2021 at 15:14

Another conjecture:

Cp. Span. ratón 'mouse', rata 'rat, Gasc. arrat 'rat', Bq. arratu 'rat'. The word occurs only in Romance, Germanic, and Celtic and is of unknown origin. It may be a Vasconic loan-word that was lost in Basque when the constraint against in initial rhotics arose. Bq. arrautu etc. were borrowed from Romance, with obligatory prosthesis because of the constraint. However, since the rat is an immigrant from East and South-East Asia (Der Große Brockhaus: s.v. Ratten), its European name too may have Asian roots. Theo Vennemann Nierfeld et al.; Europa Vasconica, Europa Semitica p.704, footenote 30 (2003)

  • Whenever the trail goes dead, somebody always blames the Basques. :) Curious indeed that Ancient Latin and Ancient Greek both lacked any dedicated term for a rat that wasn't just a big mouse. The OED3 now sports a rather long ramble on the etymology that goes a bit further than quickly blaming the millennial Vikings and leaving the matter there, but it’s still a far candle from these more recent investigations.
    – tchrist
    Dec 8, 2021 at 14:52
  • @tchrist Thanks, I'll basque in the warm glow of the ancient genomes.
    – DjinTonic
    Dec 8, 2021 at 15:11
  • It is great to see another possible route that involves Vasconic languages and Basque. The passage concludes with a similar finding in my post that it might have Asian roots. Congrats for the bounty. I've upvoted also.
    – ermanen
    Dec 22, 2021 at 4:51
  • 1
    Spanish ratón 'mouse', rata 'rat' is also slightly odd in that -ón is often an augmentative suffix, not a diminutive. Both displaced the Old Spanish mur from Classical Latin mus
    – Henry
    Nov 15, 2022 at 11:54

It may be that we would have to look for a paraphrase or a two-word expression, e.g. "great mouse". Just a vague idea for Latin: "mus + cretus" ( mouse + grown big). Perhaps one has not found more information about the origin because the idea is a word must be one word historically. That a lot of words were at first multi-word explanations that were shortened in the course of time and became a single word at the end is often not taken into consideration.

  • 4
    Huh? Are you proposing that "rattus" is derived from "mus cretus"? That seems completely random.
    – herisson
    Aug 20, 2015 at 20:35
  • Would it be so astonishing that a lot of one-part words have their origin in simple two-part paraphrases that in the course of time were melted into a one-part word?
    – rogermue
    Sep 22, 2015 at 8:19
  • The astonishing part is more that you just threw this out there. To do serious etymology work, it would be necessary to look at the history: do we have any examples of the phrase "mus cretus" ever used in Latin to refer to rats? Is there evidence of any intermediate forms, like "muscretus" or "muscratus"? Are there any other words that show a parallel shift of "e" to "a" and "t" to "tt"? If you have no answer to any of these, why do you expect anyone to be convinced by your proposed etymology?
    – herisson
    Sep 22, 2015 at 8:40
  • 1
    OK... that's true, and I'm sorry that I've been commenting from the position that this is an actual proposed etymology; if I understand right, it's more of an example of a possible way words can derive. In that case, I think it would be better if you just removed the part about "mus cretus" entirely and replaced it with an example of another word, one that is known to have derived from a two-word phrase. It doesn't have to be related to "rat." As it is, I find this answer possibly misleading (I'd worry about someone taking "mus cretus" seriously), and that's the main reason I downvoted.
    – herisson
    Sep 22, 2015 at 8:52
  • 1
    It's too bad this can't work as the origin of 'muskrat'.
    – AmI
    Oct 6, 2018 at 18:23

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