(it's) off with (one's) head

A command for one to be punished to the utmost. An imitation of an order that might have been given by a monarch or other ruler in medieval times to have someone literally beheaded. Often used humorously. I'm warning you, Phil, one more screwup like that, and it'll be off with your head! This is a cutthroat industry. If an artist can't make music that sells, then off with their head! If we don't get this report to the boss before his call with the CEO, it'll be off with our heads.