Green’s Dictionary of Slang

1. (also cosh up) to hit (with a bludgeon or ‘life-preserver’).

1896		A. Morrison Child of the Jago (1982) 47: With a sudden blow behind the head, the stranger was happily coshed.
1919		Marvel 3 Mar. 5: Give it him good and hard! Cosh the pig!
1927		E. Wallace Squeaker (1950) 164: Go and cosh him!
1931–4		D. Clemmer Prison Community (1940) 331/1: coshed, vt. To be knocked out, especially by a blow on the head.
1943		J. Phelan Letters from the Big House 149: Rothschild was boasting of having ‘coshed’ him.
1956		J. Curtis Look Long Upon a Monkey 76: He’d had a whale of a time, showing off like a stupid baby, flashing money about, nigger-rich, and the wide boys had spotted him, coshed him and rolled him for his poke.
1964		E. Huxley Incident at the Merry Hippo 210: He wasn’t cat-eyed, he didn’t know the layout of the garden, anyone could cosh him with a blunt instrument.
1973		T. Lewis Billy Rags [ebook] ‘What’s he got a riot stick for, then? You’re going to cosh us up, you bastards’.
1978		F. Norman Dead Butler Caper 59: On no less that three occasions I’d been coshed by underworld experts and [...] each floored me with a single beautifully-aimed blow.
1981		B. Max Whispers of Love 171: I was going to cosh him last night but I couldn’t get into his flat.
1999		Observer Rev. 4 July 12: Swude koshed him in the teeth with an oyster bowl.
1999		Observer Screen 7 Nov. 14: Footsteps pounding down darkened London streets, people being coshed on stairways.
2012		J. Fagan Panopticon (2013) 107: He’s a bigger cunt than she is — I wouldnae be suprised if he koshed her.

2. to break with any form of implement.

1961

(con. 1920s) J. Sparks Burglar to the Nobility 31: We copied down the addresses of empty shop then went by night and coshed in their windows [with a 14 lb. hammer].