When you're sitting on the toilet taking a shit, and your penis touches the underside of the toilet, (where all the pubic hair and pee is) thus getting the Witch's Kiss. This can happen when you get a boner or if you are just hung.
"I was watching porn the other day while taking a shit, and got a Witch's Kiss"
"I'm glad you gave me head after i got a Witch's Kiss. I felt dirty."