The criminal threatened the guard with a permanent shave.
permanent shave
idiomA throat-slitting.
Origin
disputed originFrom the early 1900s, presumably a pun on permanent wave.
In the dark alley, the thief warned his victim that he would receive a permanent shave if he did not hand over his wallet.
The gritty crime novel described how the ruthless assassin ended his target's life with a swift and brutal permanent shave.