Idiom and Slang

[billy]

i think "going to vegas to pick up girls" and phrases like it have become too main stream to be slang, for me they're idiom
in the uk it used to be "going to the dam for some brass" or "to do some brass" brass being the window girls, aka prostitutes.

maybe proto-idiom is good way of explaining it as abu points out. but for how long, pretty soon it evolves into full blown usage once enough people make it common speak.