The Collaborative International Dictionary of English v.0.48:

KO \KO\, KO \K.O.\, ko \k.o.\n.
   a knockout; a blow that renders the opponent unconscious; --
   used especially in boxing. [acronym]

   Syn: knockout.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

KO \KO\ v. t. [imp. & p. p. KO'd; p. pr. & vb. n. KO'ing.]
   To knock out; to deliver a blow that renders (the opponent)
   unconscious; -- used especially in boxing. [acronym]

   Syn: knockout.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Potassoxyl \Pot`ass*ox"yl\, n. [Potassium + oxygen + -yl.]
   (Chem.)
   The radical KO, derived from, and supposed to exist in,
   potassium hydroxide and other compounds.
   [1913 Webster]

WordNet (r) 3.0 (2006):

KO
    n 1: a blow that renders the opponent unconscious [syn:
         knockout, KO, kayo]