underbarf Share definition Flag Verb. The act of vomiting on someone's taint. Regurgitation induced from over zealous sac suckage or fellatio. Man, he was thrusting so hard I underbarfed. underbarf by tuottosjohnson February 5, 2010 0 0 Get merch Get the underbarf mug. Buy Now — $32.95 See more merch