[in imperative]informalSaid when vigorously expressing disbelief:‘Come off it, he’ll know that’s a lie.’...
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’.
My honest (and admittedly, somewhat cruel) reaction is ‘Oh, come off it, you're not that special.’