milf mustard Share definition Flag the brown yellow substance that gets on a guys dick when he fucks a milf in the ass. i know edward was hitting the milf in the ass last night, he has a milf mustard stain in his underwear. milf mustard by swampmonkey August 16, 2011 0 0 Get merch Get the milf mustard mug. Buy Now — $32.95 See more merch