come off it!

Example: “"I could beat you in a race." "Oh, come off it!"”

Origin: A shortened form of the older "come off the grass!", itself a pointed play on the common park-sign warning "keep off the grass" — telling someone pompous or self-important to step down off whatever elevated ground, literal or figurative, they'd put themselves on. The shorter "come off it," dropping the grass entirely but keeping the same dismissive punch, is documented from the late 1800s, with the sense clearly related to a companion phrase like "get off your high horse" — both aimed at deflating someone's inflated self-regard in a single blunt line.