Zipper Head

I just saw the Clint Eastwood movie Grand Torrino. It was about this racist old guy that ended up being the hero, and it was a very good movie. He referred to the Asians as Zipper Heads. I've never heard Zipper Head as a put down. Where does it come from ? Whats the origin of this racist slam ?

Originally posted by buckky
I just saw the Clint Eastwood movie Grand Torrino. It was about this racist old guy that ended up being the hero, and it was a very good movie. He referred to the Asians as Zipper Heads. I've never heard Zipper Head as a put down. Where does it come from ? Whats the origin of this racist slam ?

A search on the term produced many results. There's lots of theories.

Looks like it's a military term for how their heads looked after being machinegunned (or possibly run over by a vehicle).