Green’s Dictionary of Slang

1. (US campus) to die, to be killed [kick off v.¹ (1)].

1976		Eble Campus Sl. Nov. 3: k.o.(kick off) – to die.
1996		Eble Sl. and Sociability 36: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] KO (kick off) ‘die’.
2005		Guardian 30 Aug. 23/3: She was KO’d [...] There was blood everywhere.

2. see knock off v. (3a)