Sticking ones finger in the anus of another while they walk up the stairs. Usually accompanied by a wiggling motion and shouting "Butt Worm!"
I was walking up the steps and my girlfriend totally Butt wormed me!
A Butt Worm (or pinworm) is a parasite often found in feces, and the eggs can be spread about very easily upon contact. Having Butt Worms often leads to itchy/crawly sensations
It's a good thing I don't work in the stables, I don't wanna get any butt worms.