Race: Asians
Coined by US soldiers during Korean War. Also used in the film Full Metal Jacket. MULTIPLE reasons have been submitted:

1) If Asians were shot in the head with high-powered weapons, their heads would split as if you unzipped them

2) Vietnam War slang for "Zero Intelligence Potential" (IE: just kill them, no reason to interrorgate them)

3) Many times the Asians would be run over by military Jeeps, which left tire tracks on them that resembled zippers.

4) Asians tend to part their hair down the middle, leaving a zipper-like strip.

5) I'm not sure if the military used body bags to transfer dead enemy soldiers, but theres a possibly reference to the zipper on said bag.

6) WWII Japanese pilots wore leather helmets with zippers down the middle