an urban legend that holds that if you have vaginal or anal sex with a woman (or anal sex with a man) and punch him/her in the back of the head, it will somehow cause the vaginal/anal muscles to contract. this is medically impossible, but widely circulated among 12 year old boys and those that share their general level of maturity and/or a chauvanist animosity towards women. the only garanteed result of a donkey punch is an immediate end to your sex life and/or getting your ass kicked physically or in court.