It's in the OED! Wheee!
Advance release of additional material for SLASH n.1
orig. and chiefly Science Fiction.
[After the written form of K/S n.] A subgenre of fiction, originally published in fanzines and now esp. online, in which characters who appear together in popular films or other media are portrayed as having a sexual (esp. homosexual) relationship. Chiefly attrib.
1984 Not Tonight, Spock! Jan. 1 Recommended Book List..to include gay books, other slash zines, or media zines with good K/S stories. 1988 New Yorker 12 Dec. 38/1 ‘Spock enslaved’ is an erotic zine. It's not really a slash book, but it's part of the same movement. 1993 FRA Rev. May-June 64 There is another chapter on slash, or fanzine stories written with the assumption of a homoerotic relationship between male media characters. 1998 R. J. COOMBE Cultural Life of Intellect. Prop. ii. 128 Starsky and Hutch fans worried that public exposure of ‘Slash’ literature would hurt the reputations of stars they regarded with respect and affection.
K/S, n.
A subgenre of science fiction, originally published in fanzines and now esp. online, in which the Star Trek characters Kirk and Spock are portrayed as having a homosexual relationship; (later) any similar fiction in which a pair of (established) male characters is so portrayed. Usu. attrib. Cf. SLASH n.1
[1977 Obsc'zine Aug. 5/1, I am not trying to attack a Kirk/Spock sexual relationship in general.] 1978 Obsc'zine 20 May 89 She concentrated on the why of the popularity of the K/S relationship stories. 1986 P. F. LAMB & D. C. KEITH in D. Palumbo Erotic Universe xv. 237 Although a few isolated K/S stories appeared in earlier, general ST zines, the K/S zines as a distinct subgenre exploded onto the fan market around 1976-1977. 1995 Extrapolation Spring 45 The primacy of the emotional bond between Kirk and Spock has spawned a genre of fan fiction known as ‘K/S’ or ‘slash’. 2000 Out Oct. 36/3 Mizoguchi asserts that yaoi and, by extension, K/S and other slash fiction..are homophobic.