Hoping to understand why “fat” is used as an adjective and noun rather than “fatty” (at least in everyday English).

For example:

  • “cat fat” (noun) and “fat cat” (adjective).
  • “That is a fat cat”.
  • Not “That is a fatty cat”.


  • “cat skin” (noun) and ”skinny cat” (adjective).
  • “That is a skinny cat”.
  • Not “That is a skin cat”.
  • Also not a “hair cat”, “fur cat”, “scare cat”, “smell cat” &c.

Also consider the the verb form:

  • “there is more than way to skin a cat”.
  • one would “fatten a cat” and not “fat a cat”
  • you can also “scare a cat”.
  • 1
    The adjective fatty nearly always refers to meat; if you were eating a cat, you could say "this cat is very fatty" if the meat contained a lot of fat.
    – Showsni
    May 18 at 13:18
  • fatty has two distinct meanings: one refers to the fat content of food, the other is either an insult or cutesy. My cat is such a fatty. And she doesn't even eat fatty foods. Fatty Arbuckle was a famous silent screen actor arrested for murder.
    – Lambie
    May 18 at 15:26
  • 1
    It's worth pointing out that fat (adj.) goes back to Old English, while fat (n.) appeared only in the 14th C, which would be Middle English. Both derived from a verb form meaning to stuff or cram. (source: Etymonline) So "fat" the adjective simply doesn't derive from a noun form like "skinny" or "hairy" do, it existed before the noun homonym did.
    – The Photon
    May 18 at 16:11

1 Answer 1


The suffix "-y", applied to a noun, means "like" or "full of". So "grassy" means either "grass-like" or "covered with grass".

The word "fat" does double duty as an adjective "having excess flesh" or noun "substance under the skin of animals". "Fatty" is a modification of the noun, so something that is "fatty" has lots of the substance "fat".

"Fat" and "fatty" therefore strictly mean different things. There may be an overlap - a "fatty" cat may also be "fat" - but it isn't necessarily the case.

We can call an overlarge cat "fat" without having to worry about whether the large size is due to the actual substance 'fat" (which would be necessary for it to be "fatty").

In your other examples: a "cat skin" refers only to the skin, not to the cat. A "skinny cat" is a cat with lots of skin (although skinny has mutated into a word meaning "thin" or "without much flesh" rather than the strict meaning implied by the suffix). A hairy cat has lots of hair; a furry cat has lots of fur etc. There is no corresponding adjectival meaning of "hair", "fur" etc. so saying "hair cat" or "fur cat" is wrong.

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