Slow has the adverb slowly. I tend to use fastly as the adverb for fast.

However, it is underlined in most spell checkers I use, which makes me wonder about the existence of this word.

Is fastly a correct word? If not, what should be used instead, and why is that different from its antonym?

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    No need to apologize, this site is for people like yourself that are interested in improving. The sad fact is that many native English speaks don't understand "trivial" concepts, and don't care about being correct. You should be proud of the fact that you even asked. Aug 20, 2010 at 1:29
  • 4
    Those native English speaks ought to be ashamed of themselves.
    – delete
    Aug 20, 2010 at 2:01
  • 2
    Check out this video from Merriam-Webster on flat adverbs: merriam-webster.com/video/…
    – user13575
    Jan 9, 2012 at 18:23
  • Related.
    – tchrist
    Jan 28, 2023 at 16:08

9 Answers 9


There is no need for "fastly" because "fast" is both an adjective and an adverb. So, "I ran fast" is completely correct.

The existence of "fast" as an adverb does not preclude the future development of a word "fastly", but it does hinder it.

One might note that the corresponding adjective "slow" does take the -ly suffix, but this has no impact on the behavior of "fast". (There is also no real reason why, for example, we have warm/warmth, but cool/coolness. Semantically related things sometimes have similar morphological patterns, and sometimes not.)

One interesting thing worth noting (that was brought up in a comment by Jimi Oke) is that there are cases of adjectives with identical adverbs that also have an -ly form; for example, we have "right" and "wrong" as adjectives and adverbs, but we also have "rightly" and "wrongly". In such cases, the -ly form has carved out its own semantic niche; the adverb "right" and the adverb "rightly" cannot be used interchangeably in every situation. I can say "turn right" and "rightly so", but I can't exchange them in either sentence.

With normal adjectives that cannot become adverbs without -ly, usually the -ly just transparently makes the adjective adverbial — it doesn't have its own separate semantic nuances (e.g. "quickly" simply means "in a quick manner").

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    Er...I beg to differ. I drove quickly, but the car was fast. Fast is an adjective not an adverb. The adverb is quickly. Aug 19, 2010 at 22:24
  • You could, however, be stuck fast. I've no idea what part of speech 'fast' is in that context. Aug 19, 2010 at 22:25
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    As I said, "fast" can be an adjective or an adverb. "The car was fast. The car drove fast." Both are correct. You can even check the dictionary if you don't trust me: dictionary.reference.com/browse/fast .
    – Kosmonaut
    Aug 19, 2010 at 22:36
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    Imagine if a pedant insisted that you could not say "How fast does this car go?" because you ought to say "How quickly does this car go?" I think "quickly" actually sounds silly there.
    – Kosmonaut
    Aug 20, 2010 at 20:28
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    @jjackson: I am 100% certain. Any dictionary will back this up.
    – Kosmonaut
    Jan 28, 2011 at 1:56

The adverb form of fast is irregular. It is one of several exceptions, as is "well" as mentioned by Dena A.

a fast runner   /    run fast
a hard worker   /    work hard
a bad smell     /    smell bad

etc. etc. etc.

About work hard: 'he works hard', in contrast to 'he is hardly working'. Two different meanings.

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    Small note... bad is not an irregular adverb in this case, it is an adjective with irregular verb behavior (see rule 2 of grammarbook.com/grammar/adjAdv.asp). You could say "smell badly" to talk about being unable to smell properly.
    – Kosmonaut
    Aug 21, 2010 at 20:09

Fast is an adverb as well as an adjective, so you wouldn't use fastly. Another common adverb that doesn't follow the pattern of ending in ly is well, not the expected goodly, which is actually an adjective and means a large quantity, e.g. a goodly sum.

  • Love and lovely follow this pattern as well I guess. (Not too sure, though.) Love is a noun/verb and lovely is an adjective. Jul 5, 2017 at 8:08

Per etymonline, fastly is the 'former adverbial form of fast (adj.), from O.E. fæstlic "firm, fixed, steadfast, resolute;" obsolete in 19c., simple fast taking its place.'

There you go folks, so much for logic...


Though word fastly used in many places (you can find many results by searching Google) but still it is not correct.
For reference see the definition of word fast from WikiDictionary

See the post wrong / fast-- adverbs with no 'ly' ending to learn more.


This reminds me of how small children extrapolate grammatical constructions in seemingly logical ways, except English is not always that logical.

So my daughter (3¾) will correctly say "I colour in neatly" or "I'll do it carefully", but then also "It comes lastly" "I can hop bigly" "Go farly" "Squeeze me hardly" etc.


Oh, fastly is a word alright. It just happens to be an obsolete one.


"Fast" can also mean to hold onto in a strongly, and "fastly" is the adverb of that word. E.g. "The crew held fastly onto the hand-rail of the boat as it rolled in the storm"

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    I wouldn't say that, and google gives 118 hits for "held fastly", as against 298 000 for "held fast". Another adjective which is also an adverb is "hard". "Hardly" exists, but has a completely different meaning.
    – Colin Fine
    Aug 20, 2010 at 12:24
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    Sorry, but "fastly" just isn't a word. You drive fast. You hold fast to the handrail of the boat. You hold fast to an idea. You can also drive quickly, but that's the same as driving fast.
    – Dena A
    Feb 7, 2011 at 18:50
  • As said from Kosmonaut too, fast is already an adverb, and the suffix -ly is used to create an adverb from an adjective.
    – apaderno
    Feb 7, 2011 at 18:51

This is emblematic of a misunderstanding that gives rise to forms truly abominable, begetting debellished hypercorrections thusly seen in casual prose all over the internet todaily.

The problem is that people oftenly get this mistaken notion that adverbs need to end in ‑ly. I know no likely explanation for this blunder, but they stilly do it anywaily.

Howeverly certain they are of this non-rule, nothing could be furthlier from the truth, and the soonlier they realize this, the morely their writing stands to gain; it sure won’t happen beforely.

This semisilly answer is intended to come across friendlily, not meanly or worsely.

  • Ha ha ha ha! Awesome answer! (I know comments like this are against the rule, but I just couldn't help appreciating your witty answer!) Jul 5, 2017 at 8:12

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