Green’s Dictionary of Slang

1. to exact an extra-legal ‘punishment’ esp. beloved of, and poss. introduced by, the IRA, whereby victims are shot through the kneecaps and, while painfully crippled, are not actually killed.

1974		Times 20 Nov. 16: An indication of the brutality which can be used in the name of internal discipline was provided recently when a man from the Shankill Road was ‘kneecapped’ with a Black & Decker electric drill.
1978		New Yorker 12 June 60: I.R.A. ‘hit men’ punish detractors and defectors, by subjecting them to ‘kneecapping’. ‘It used to be done with guns [...] but lately they’ve been running an electric drill through the knee in such a way that even after medical treatment there’s a peculiar, easily identifiable walk.’.
1989		in G. Tremlett Little Legs 105: I wanted to knee-cap him.
1999		Guardian Editor 16 July 16: The Waterside Unit knee-capped Culm Michael and he bled to death.
2006		C. Hiaasen Nature Girl 259: He was serving twenty to life for kneecapping two associates.
2020		(con. 1991-94) W. Boyle City of Margins 14: Big Time Tommy saying it’s time to kneecap the guy if need be. A kneecapping’s the beginning. Then both arms get busted.

2. to break someone’s kneecaps with a stick or similar weapon.

1989		J. Morton Lowspeak.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 76: I look back on the jelly beans I have shot, stabbed, bashed, iron-barred, axed, knee-capped, toe-cut, blow-torched, killed and generally up-ended.
2001		N. Griffiths Sheepshagger 11: Any excuse to kneecap a bailiff, mun, any fuckin excuse.
2006		G. Malkani Londonstani (2007) 16: His seat pushed all the way back so I was gettin, like, kneecapped.
2013		T. Piccirilli Last Whisper in the Dark 237: ‘He’s got fifty legbreakers [...] if he just wanted to kneecap her’.

3. in fig. use, to destroy someone’s career.

1987

R. Campbell Alice in La-La Land (1999) 129: His foot was on the ladder. All he had to worry about was that nobody should take the notion to kneecap him and leave him for dead.