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1.
A fart that happens so fast and unknowingly that there is no time to tighten butt muscles.
Usually by constant laughter
John: So the police officer says 'thats not a donkey, thats my grandma!'

Jerry: LMFAOBBQWTFLOL!11! *flash fart*

John: Yo did you seriously just flash fart?
by Cody Baggs is here October 14, 2007