(1)A chin punch
(2)A slight nod of the head where the head is quickly tilted up, then slowly lowered. typically used as an inconspicuous nod of recognition or an attempt to get someone's attention.
To punch with the chin
(1) When I was fighting that guy and he had my arms pinned, all I could do was gremp him in the face.
(2) Dude, do you see that guy with the beany over there? He just gremped at me.