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The act of not fully wiping after dropping a duece and then commencing to fart in a persons general direction. sending tiny feces shrapnel everywhere.
Holy shit! He just left off a huge butt grenade.

look out, a butt grenade!
by Str8sk8r June 14, 2010
an explosion in your asshole :)
dude, i need a new toilet, i had another butt grenade go off!!
by the muffin man :) January 24, 2009