A man who was pleasuring himself in public had an unhappy ending yesterday, when an offended onlooker whacked the auto-stimulator in the head.

According to the San Francisco Police Department, the 21-year-old man was on a center island bus stop at Church and Market Streets at 11:55 a.m. Sunday, "masturbating in public."

A 35-year-old bystander "became angry watching" the scene, according to the SFPD, and approached the meat beater.

A "verbal altercation ensued," followed by a physical one. Eventually, the anti-jerker hit the pro-jerker in the head with a "metal crow bar."

The formerly masturbating man was knocked unconscious by the blow.

According to SFPD, the injured man was transported to the hospital for a non-life-threatening injury to his head. His assailant, police say, shot off following the attack and has not been arrested.