When someone comes up from behind you and puts their arms around you as to hug you and then adds a pelvic thrust into your lower region.
We were in the garage and thought nobody was looking so I surprised my wife with a humphug...unfortunately our son caught us and asked what we were doing.
When you aproach someone in order to give them a friendly hug, and that person (hopefully of the opposite sex) practically sits on your lap. Commonly found in desperate people.
"hey you know that lisa chick? yeah, i hugged her and she gave me the time of my life! what a hump hugger."