When one heterosexual man (Man A) runs up behind another (Man B) , jumps, pushes of the Man B's shoulders, and humps the Man B as high up on his back as possible. This is usually done to show Man A's dominance over Man B, or to make Man B subservient.
Rob: Dammit Steve! Quit Man Humping me! It's annoying as hell!
Steve: Maybe I would if you weren't such a little bitch.