I knew 'zipperhead' was a racial slur for asians, but I didn't know why, so I decided to look it up on urban dictionary. This is what it said:
"A derogatory term used in reference to people of Asian descent.
It is said to have been coined during the Korean war by frontline troops whom had run over enemy troops in jeeps.
The soldiers claimed that the tire tracks from the jeeps left a pattern resembling that of a closed zipper along the corpse.
Damn, look at all the zipperheads here, it's like they've just taken over."