Green’s Dictionary of Slang

a knife slash that runs from the corner of the mouth up and across the cheek.

1999		‘English Sl.’ at BBC.co.uk 🌐 Chelsea smile – The scar you get when you have been cut ear to ear is known as a Chelsea smile. Comes from the criminal underworld of the 60s, the Kray era.
2002		Daily Mirror 6 Aug. 🌐 Sarah Payne’s jailed killer is scarred for life after a fellow inmate slashed him in the face with a home-made blade. Hated paedophile Roy Whiting, 42, was standing on a landing when he was grabbed and had both corners of his mouth savagely cut in a wound known to cons as a ‘Chelsea smile’.
2002		T. Ferguson ‘Adventures in the Land of Mr Sparkle’ on Virtual Todd Ferguson 🌐 Week Eighteen 25 Nov–1 Dec. [Internet] Where are they when we’ve lost the last five or six games and are playing like crap? Wankers, bloody wankers. They deserve a Glasgow Kiss or a Chelsea Smile!
2005		www.jimnshelle.net 9 Dec. 🌐 I saw someone do a Chelsea Smile once and I threw up right then and there on the spot.
2009		T. Black Gutted 17: A wince stretching out his half-Chelsea smile.