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]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "KO":
   Cadmean victory, Pyrrhic victory, anesthetize, ascendancy, bedaze,
   benumb, besot, blackout, blunt, call off, cancel, catalepsy,
   catatonia, catatony, championship, chloroform, clincher, coldcock,
   coma, complete, conquest, cook, crusher, deaden, death stroke,
   deathblow, defeat, delete, desensitize, dish, dispose of, do for,
   do in, dope, drop the curtain, drug, dull, easy victory, end off,
   end-all, ender, etherize, expunge, extinguish, faint, final stroke,
   finalize, finish, finisher, finishing stroke, fix, fold up, freeze,
   get it over, get over with, get through with, give the quietus,
   grand slam, grayout, kayo, kayo punch, kibosh, kill, knock out,
   knock senseless, knock stiff, knock unconscious, knockout,
   knockout blow, landslide, landslide victory, last dab, lay out,
   lipothymia, lipothymy, mastery, moral victory, narcotize, nirvana,
   nirvana principle, nothingness, numb, oblivion, obliviousness,
   obtund, palsy, paralyze, perfect, picnic, polish off, pushover,
   put paid to, put to sleep, quietus, runaway victory, scrag,
   scuttle, semiconsciousness, senselessness, settle, settler,
   shoot down, sink, sleep, sockdolager, stopper, stun, stupefy,
   stupor, subdual, subduing, success, swoon, syncope, torpedo,
   total victory, triumph, unconsciousness, undo, victory, walkaway,
   walkover, win, winning, winning streak, wipe out, zap