Definition of fat adjective from the Oxford Advanced American Dictionary

      

    fat

     adjective
    adjective
    NAmE//fæt//
     
    (fatter, fattest)
     
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  1. 1 (of a person's or an animal's body) having too much flesh on it and weighing too much a big, fat man/woman You'll get fat if you keep eating so much junk food. He grew fatter and fatter. fat, flabby legs opposite thin
  2. 2thick or wide a fat volume on American history
  3. 3[only before noun] (informal) large in quantity; worth a lot of money a fat sum/profit He gave me a nice fat check.
  4.  
    noun [uncountable] Fatness tends to run in families.
  5. Vocabulary Buildingsaying that someone is fat Fat is the most common and direct word, but it is not polite to say to someone that they are fat:Does this dress make me look fat? You’re looking fat now. Overweight is a more neutral word:I’m a little overweight.It can also mean too fat, especially so that you are not fit. Large or heavy is less offensive than fat:He’s a fairly large man.Big describes someone who is tall as well as fat:Her sister is a big girl, isn’t she? Plump means slightly fat in an attractive way, often used to describe women. Chubby is used mainly to describe babies and children who are fat in a pleasant, healthy-looking way:the baby’s chubby cheeks Tubby (informal) is used in a friendly way to describe people who are short and round, especially around the stomach. Stocky is a neutral word and means fairly short, broad, and strong. Stout is often used to describe older people who have a round and heavy appearance:a short, stout man with a bald head Flabby describes flesh that is fat and loose:exercises to firm up flabby thighs Obese is used by doctors to describe people who are so fat that they are unhealthy. It is also used in a general way to mean “really fat.”Note that although people talk a lot about their own size or weight, it is generally not considered polite to refer to a person’s large size or their weight when you talk to them or about them.Word Familyfat adjectivefatty adjectivefatten verbfattening adjective
    Idioms
    (a) fat chance (of something/doing something)(informal)
     
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    used for saying that you do not believe something is likely to happen “They might let us in without tickets.” “Fat chance of that!”
    a fat lot of good, use, etc.(informal)
     
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    not at all good or useful Paul can't drive, so he was a fat lot of good when I broke my arm.
    it's not over until the fat lady sings(saying)
     
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    used for saying that a situation may still change, for example that a contest, an election, etc. is not finished yet, and someone still has a chance to win it
See the Oxford Advanced Learner's Dictionary entry: fat