This term was started during the Vietnam conflict to describe a Vietcong tactic.
After killing a GI in combat, the Vietcong split a soldier's head with a machete, then insert a live hand grenade with the pin removed. Replacing the helmet hides the booby trap.
When medics come to recover the dead, moving the body will cause the helmet to fall off and release the grenade, killing or maiming anyone in the immediate area.
Zipperhead describes the mutilation that the Vietcong inflicted on fallen soldiers.
Fuckin Zipperheads got another one of our boys.