Pustachio Share definition Flag A guy that is too young to grow a mustache but grows a lame ass one anyway. Did you see that fuzz on Aaron's upper lip ? He's 12, it looks like he glued pubes to his lip. . . a pustachio !!! Pustachio by MTS73 August 25, 2010 0 0 Get merch Get the Pustachio mug. Buy Now — $32.95 See more merch