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As well as being an adjective, fast is an adverb. We use it all the time as such:

He ran fast.

However, though slow is definitely an adjective, it sounds wrong when used as an adverb, because slowly is more common.

He ran slow.

We would always say:

He ran slowly.

My question is, why isn't fast treated the same way as slow; why is there no fastly, while there is a slowly?

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  • Related: Is "fastly" a correct word?
    – Kosmonaut
    Jul 4, 2011 at 2:02
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    Shouldn't this be titled "Why is 'fastly' not a word?" (with no change in any answers needed).
    – Mitch
    Jul 4, 2011 at 22:27
  • Most adjectives have adverbial forms, so I agree with Mitch in the opinion that this question would be more useful if it were about "fastly".
    – Anicul
    Nov 8, 2011 at 6:22
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    Slow can be an adverb, too: "My watch runs slow." And fast isn't as much of an adverb as quickly. I would say "He is running fast", but I wouldn't say "He is running fast to the store". And Google supports me--"ran fast" and "ran quickly" are roughly comparable in frequency, but "ran fast to" is much less frequent than "ran quickly to".
    – Peter Shor
    Nov 8, 2011 at 23:04
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    @drɱ65: Yes. Thank you for editing! I removed my downvote. Also, I agree with Peter in the opinion that fast is only an adjective.
    – Anicul
    Nov 9, 2011 at 1:05

5 Answers 5

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Because real, living languages have quirks and are never entirely systematic. The "ly" ending is generally indicative of an adverb in English, but not all adverbs end in "ly", and not all words ending in "ly" are adverbs.

Absolute consistency in a language is an indication that the language is a pidgin, a relatively new creole, or an artificial language (like Esperanto or Volapük). Any language that is in everyday use and has been around long enough to gather scratches and dings, will have them.

Quick and fast are both words that may or may not be related to rapidity. In their adjective forms, quick also means living, and fast can mean secure or firm. As an adjective or an adverb having to do with speed, fast seems to have been the result of some strange semantic drift in the period leading up to Middle English.

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  • 3
    Sir, there isn't a single answer of yours I wouldn't like. You seem like a well educated person and I'm very happy that you're a part of this community. +1 !
    – Frantisek
    Jul 4, 2011 at 2:25
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    Thanks, but now my hat won't fit, and I need to find other people to share the burden of carrying my ego around :)
    – bye
    Jul 4, 2011 at 2:34
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    In fact, English has acquired so many scratches and dings that it has died. Death occurred on August 21, 2010, and was reported by The Washington Post, where, indeed, the language gave its death rattle. Here is the link to the obit: washingtonpost.com/wp-dyn/content/article/2010/09/13/…
    – Hexagon Tiling
    Mar 31, 2012 at 13:02
  • That's funny, and I'm sure "tongue in cheek". Probably most users of this site have a usage, or a list of usages, that makes them squirm [mine's confusing "of" with "'ve", as in "He shouldn't of."] But English will change and survive.
    – David Garner
    Jan 29, 2015 at 14:03
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From etymonline:

fastly `(adv.): "quickly," c.1200, former adverbial cousin to fast (adj.), from Old English fæstlic "firmly, fixedly, steadfastly, resolutely;" obsolete in 19c., simple fast taking its place.

So it was in fact a word, but has fallen out of use.

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  • Please provide the link to the references you provide.
    – Neeku
    Jan 29, 2015 at 12:54
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I think this is a special case of the recent discussion of adjectives being substituted for adverbs. As a British ex-pat living in the US when I hear "He ran fast", I hear a bit of that American twang, even though Brits might also say that. I think though that Brits would feel it a little bit of an awkward phrase, and might choose "He ran quickly", but only because their is no appropriate adverbial form of fast. I should say that I think "he ran quickly" doesn't mean exactly the same as "he ran fast" but it is close enough for government work. I should also say that some might say that there is an adverbial form of "fast", being "fast", but that rather begs the question, don't you think?

When I hear "he ran slow" I hear that American twang even more strongly, and would definitely substitute the adverb.

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    It's pretty common nowadays in even only slightly informal speech to drop the '-ly' in adverbs. It may sound "twangy" but I don't think it is is restricted to any particular AmE regional dialect.
    – Mitch
    Jul 4, 2011 at 13:46
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    The Etymonline entry for 'fast' rather thoroughtly disagrees with you about this. According to it, 'fast' in the sense of 'quick' developed as an adverb from Old English or Old Norse, well before the founding of America.
    – user1579
    Jul 4, 2011 at 18:31
  • As an American, I do perceive a difference between "fast" and "quickly". The phrase "He spoke fast," would usually mean he is speaking with high speed, whereas "He spoke quickly", would usually mean "He spoke right away". (Each adverb can take both meanings: "he acted fast" usually means "right away", whereas "he ran quickly" usually means with "high speed".)
    – Peter Shor
    Nov 8, 2011 at 12:11
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Well, there is always quickly and rapidly working as equivalents of fast in terms of velocity or speed if you insist on the suffix-ly.

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    I'm sorry, but in both your examples, "slow" is an adjective.
    – Daniel
    Jul 4, 2011 at 2:31
  • I updated my answer, and yes I know they are adj. but I provided them as adverbs. I removed the examples so no one will confuse.
    – Jamie
    Jul 4, 2011 at 18:25
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The answer is that "slowly" was not the original form. "Slow" is an adv. just as "fast" is. "Slowly" was introduced by classroom instruction that attempted to induce "ly" endings for adverbs as standard.

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    And when would this classroom instruction have happened? Shakespeare (who wrote his plays well before grammar police started infecting classrooms) used both fastly and slowly.
    – Peter Shor
    Mar 4, 2012 at 23:57
  • What utter nonsense. Slowly as an adverb is attested in English as far back as the ninth century, about six hundred years before slow as an adverb is attested.
    – Janus Bahs Jacquet
    Jan 29, 2015 at 12:43

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