Guppy

The act of placing your hand between someones ass cheeks from behind, and flopping your hand side to side in a manner that resembles a guppy. One must shout "guppy" before executing the act; Forewarning is necessary to allow the victim time to clench cheeks and minimize penetration.

(noun) "I let my guard down while Jennie was behind me and she gave me a guppy."
(verb) "She guppied me so hard, I'll be walking with a limp for the next 24 hours."