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 them2) 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, wh...