The act of running up behind someone, reaching high between their legs (almost but not quite in the crotch seam) and slapping one's hand back and forth as if to motorboat
the upper thighs with the hand. The recipient of the twaffle is now "it" and must twaffle someone else to continue the game. Verb, origin Grafton VT Sept. 2010.
I'm going to twaffle the bride right before she gets her photo taken.