insta flop Share definition Flag The sensation of your erection instantly dropping when you see a particularly ugly female species. Can be used with the phrase boom. I saw Helga earlier after getting with Amy, she wasn't wearing underwear. BOOM, insta flop! insta flop by Kempo May 9, 2006 0 0 Get merch Get the insta flop mug. Buy Now — $32.95 See more merch