Fat Flapping 

The action of getting volatile and angry over a minute and niche thing of something that's praised.

Friend 1: Hey, did you hear about that new Left 4 Dead 2 update?
Friend 2: Yea, I hear it's awesome! Though I did hear one dude complain that the update ruined his mods.
Friend 1: Don't mind him, he's probably just some fat flapping weeb who's upset that his 150 Hatsune Miku mods don't work no more. Let's go play!
Friend 2: Ight, bet!

Fat Flapping by D.E.E.M.M.3 September 27, 2020

Big, fat, flapping rat trap 

The mouth someone who is saying something very stupid

Ian: I know everything
Tea-Lady: Shut your big, fat, flapping rat trap.

Big, fat, flapping rat trap by Craig Cummings October 7, 2005

Fat Fapping 

To watch your arm jiggle while you masterbate.

Hey what did you do last night? "Fat Fapping" you mean when you stare at your arm while you masterbate? Yupp.

Fat Fapping by remarcnoremac November 10, 2010