Not so much an expression but an insult.
I hate the term "Gammon" used as an insult.
Being Jewish I find it slightly distasteful, but could live with that alone - the world doesn't revolve around me and mine.
Direct an insult at anyone else based on their appearance and you'd be accused of being an "ist" of some kind, and rightly so, and this is really no different.
And on top of all that it simply playground level juvenile name-calling. The type that trot it out think they're clever and cackle along with their narrow minded friends, when all they're doing is celebrating puerlie behaviour that they themselves would condemn in any other context.
It makes my skin crawl.