When a person, usually a boy, stands infront of the victim, with the victims head at penis height, grabbing the back of the head and pumping twice, thrusting the penis into the face of the victim, whilst saying "UNSA UNSA".
This usually causes great embarassment, and ones face is left feeling rather defiled.
Dan: Damn, I just got head humped!
Paul: Oh *Precedes to head hump dan* UNSA UNSA
Dan: F*%KING HELL