Mr. Roundmouth Share definition Flag Mr. Roundmouth is a slang term for your butthole. Dang dude, I am never drinking that much again. Mr. Roundmouth was puking all morning and I couldn't get any sleep. Mr. Roundmouth by Willy Wig January 11, 2012 0 0 Get merch Get the Mr. Roundmouth mug. Buy Now — $32.95 See more merch