A nickname for the form of torture that involves cutting a victim's face from the edges of the mouth to the ear, the cut, or its scars, form an "extension" of what resembles a smile. Usually the victim is given a small cut, around 1/4"-1/2", in the corners of the mouth and is then further injured by kicking, stabbing, or other means, causing them to scream and the cuts to further tear. It is also known as a Glasgow grin, Chelsea smile
, or Chelsea grin
Notably the Black Dahlia Murder, in which the victim was cut in half at the waist and given a Glasgow Smile. But maybe not in that order.