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.