come off it

[come off it] also [get off it] {v. phr.}, {slang} Stop pretending; bragging, or kidding; stop being silly. - Used as a command.

"So I said to the duchess..." Jimmy began. "Oh, come off it," the other boys sneered.

Fritz said he had a car of his own. "Oh, come off it," said John. "You can't even drive."