Green’s Dictionary of Slang

(a) (also kay) a kilogram of any illicit drug.

1992		A. Duff One Night Out Stealing 30: A massive celebration party with half a k of dope.
2000		J.J. Connolly Layer Cake 1: It’s [i.e. some cocaine] about forty kay or twelve years’ worth, depending how you look at it.
2000		P. Temple Dead Point (2008) [ebook] The Feds helped the boys bring in more than the two Ks they find on them.

(b) ketamine hydrochloride, a mildly hallucinogenic drug, developed as a battlefield anaesthetic, associated chemically with phencyclidine (PCP) and often used as a legal substitute for MDMA; thus K-head, an habitual user of that drug.

1992		G.P. Pelecanos Firing Offense 210: The K-heads and cocaine kids moved about these rooms like hopped-up insects.
1997		New York Mag. 8 Dec. 49/4: Somebody gave me a bump of coke, but it turned out to be K.
2001		Guardian Guide 27 Jan.–3 Feb. 28: The original apocryphal K story was the girl who fell asleep in a waltzer, hired by the club from a fun fair.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 97/1: K n. ketamine.
2003		Indep. Rev. 7 Mar. 4: Ecstasy is out, in favour of a horse tranquilliser that makes you feel, to quote one K-head I spoke to, ‘just like you’re dead.’.

(c) phencyclidine [? mis-reading of sense 3b].

1998		(con. 1986) G. Pelecanos Sweet Forever 11: He’d lost too many amigos to that stuff, K-heads who had dropped their bikes doing eighty, or taken on the wrong guys in bars.
2001		G. Pelecanos Right As Rain 50: Ray had been paid to kill some K-head who’d ripped off the stash of a dealer.