saggy bum Share definition Flag A boy with a cute, non existant or flat bum. Usually an athletic and attractive teenage boy. "omg , look at luke's ass!""yeah, i know, what a saggy bum" saggy bum by Asses are us. November 30, 2009 0 0 Get merch Get the saggy bum mug. Buy Now — $32.95 See more merch