Meaning "to masturbate", it is derived from the following
joke.
An owner of a bakery was an avid
cake-
baker , but her three children never failed to steal and eat the
cake before it could be used for its intended purpose (To be sold. Duh). The woman decided to find out who was eating these cakes. She included a few tablespoons of metal BBs (the kind that go in Airsoft guns) into the cake batter.
Later that day, the baker was approached by her (very freaked) daughter. "Mom, mom, I went to the bathroom and I pissed out BBs!"
And so the mother
beat her daughter.
A few hours later, her second daughter ran up to her and repeated the exact same thing as the first daughter.
And so the mother
beat her daughter.
After dinner, her son ran up to her and said, "Mom, mom, I was down in the basement jacking off, and I shot the cat!"