Definition of come off it in US English:

come off it

phrase

informal
  • in imperative Said when vigorously expressing disbelief.

    • ‘Oh, come off it, it's true that they can be justly blamed for all sorts of devilish chicanery, but your presumption is crazy.’
    • ‘Indeed, she claims that there is an unspoken English rule that she calls ‘the importance of not being earnest’, along with a peculiarly English injunction to say, ‘Oh, come off it!’’
    • ‘Come off it, that's not something ‘worth remembering’.’
    • ‘I can accept there would be a little disappointment associated with a camp designed for children's activities being cancelled but come off it, surely the child could make do with either the swimming pool or the beach.’
    • ‘Well, I say hooray for the older man, too, but come off it.’
    • ‘My honest (and admittedly, somewhat cruel) reaction is ‘Oh, come off it, you're not that special.’’
    • ‘‘Oh come off it, mate,’ he said, because he is not only a hawk, but has a keen and impatient mind.’
    • ‘OK, there are bound to be borderlines for teenagers - but come off it.’
    • ‘‘Oh, come off it,’ I said, when they started raving.’
    • ‘So everything I do, there's this little bit of me that's saying, Hey, come off it, you can't do this.’