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1.
Literally, your butt. Or bottom, booty, bum, rear, buttocks, derrière, etc.
Kelsey: Dude, don't be checkin' out my fessis!
Philippe: I'm sorry, I couldn't help it!
Kelsey: okay, well, don't let it happen again or i'll have to kick your fessis.
by Kelseysomethin' April 01, 2009