A Galumpkin is a sexual act, where a male (sometimes mentioned as the "galumper") lathers his testicles in peanut butter and spreads grape, or strawberry, jelly on his perineum (also referred to as a gooch) . He then jumps repeatedly, to make the testicles clap and mix the ingredients. From here another, typically man, will eat it as if it was a peanut butter and jelly sandwich.
"I heard that John was giving out Galumpkins, I had no clue he was a Galumper"
"I saw two guys Galumping"