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1. If you have a fat mouth, you talk too much, especially about things that should be secret.

2. Someone that's very talkative, especially when boastful.

3. One that informs, especially against another/others.

4. Someone who talks too loud and boastful; whoride.
That chick's a fat mouth! Don't tell her anything!
by Mr. Terrence L. Trezvant April 07, 2007
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Used to describe someone who cannot shut up, talks with verbal diahorrea or who swears a lot- almost used in that case interchangeably with 'foulmouth'.
She's such an anonying little fatmouth.

Shut up you fatmouth little bitch!
by whitecracker556 March 08, 2009
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1. the kind of person that talks crap and may have pathetic comebacks

2. someone who spreads rumors, even if they aren't true

3. that one person you can't tell your secrets to
Boy: You're a freak and you're ugly.

Girl: Kid, you need to back off and leave me the hell alone before I freaking punch you in your big, fat mouth.
by BatmansGotSwag October 18, 2011
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someone who runs their mouth and talks all the time.
you wouldnt want to tell them anything, because they are likely to tell the world.

watch your back around a fat mouth.
People named RJ tend to be a liar and a fat mouth.
by Jaeeeeeeeeeeeeyy October 10, 2010
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A label for politicians (and people with white collars and parted hair) who spray unnecessary verbosity and excrete meaningless Wordshit, while failing to perform their duties. Applies to 99.9% of modern politicians.
Trump: "Please do not reduce me to vapid innocuous euphemisms."
Other useless politician: "Oop, we've got a Fatmouth on our hands".
by Alexander Superslim October 10, 2017
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