"Get out of the town"
why don't I find the above sentence in any books of idiom, is it an idiom?
Not that I know. You could say Get out of town to tell someone to leave a place.
Or to express incredulity, usually of a positive nature.
"I made a million dollars in the stock market last year."
"Really? Get out of town!"
You can also say "Get out!"
It was a running gag on Seinfeld
YouTube - GET OUT!!