When two fully clothed (non) gay men kiss and frolick with one another in a public display of animalistic psuedo-affection.
Chris got so liquored last night he attacked that unsuspecting fellow in the soft hat and started man humping him on the ground.
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.