A sudden, unsuspected violent attack to someone's face using your privates as the weapon of choice. Usually done when a victim's face is exposed and vulnerable to attack.
The phrase is used as an insult or an interjection expressing disgust or anger. Derived from the Jamaican word bomboclaat, bomboshart is a combination of bomboclaat and shart.
Joe: Hey, where have you been all day?
John: Just finished up in the washroom.
Joe: “BOMBOSHART”