the idiom is 'come off it', and is used when a person is saying something far-fetched or not believable.
John:"I can't ask her for a date. She'd never go out with me."
Pete: "Come off it mate. She went out with Ian and you're better looking than he is."
(Oh, adolescent males!)
He: "I get 45 miles to the gallon in this little beauty."
She: "Come off it. My car has a smaller engine and even I only get 35."