The Collaborative International Dictionary of English v.0.48:

Knock-out \Knock"-out`\, a.
   That knocks out; characterized by knocking out; as, a
   knock-out blow; a knock-out key for knocking out a drill from
   a collet.
   [Webster 1913 Suppl.] knockout

The Collaborative International Dictionary of English v.0.48:

knockout \knock"out`\, knock-out \knock"-out`\, n.
   Act of knocking out, or state of being knocked out; the act
   of rendering a person unconscious by a blow. [WordNet sense
   1]
   [Webster 1913 Suppl. + WordNet 1.5]

   2. a blow which causes a person to become unconscious.

   Syn: knockout blow, knockout punch.
        [WordNet 1.5]

   3. (Boxing) the winning of a boxing contest by rendering the
      opponent unable to stand for a specified period, usually a
      count of ten; -- in contrast to a win by a decision; as,
      Muhammed Ali won by a knockout in the first round.
      [WordNet 1.5]

   4. a strikingly beautiful woman. [Informal]
      [PJC] knockout drops

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "knockout":
   Cadmean victory, KO, Miss America, Pyrrhic victory, ace,
   ascendancy, bathing beauty, beau ideal, beaut, beauty,
   beauty contest winner, beauty queen, belle, blackout, bunny,
   catalepsy, catatonia, catatony, championship, charmer, clincher,
   coma, conquest, corker, coup de grace, cover girl, crackerjack,
   crusher, daisy, dandy, darb, dazzler, death stroke, deathblow,
   dilly, dream, easy victory, enchantress, end-all, ender, eyeful,
   faint, final stroke, finisher, finishing stroke, grand slam,
   grayout, great beauty, hit, honey, humdinger, kayo, kayo punch,
   killer-diller, knockout blow, lady fair, landslide,
   landslide victory, last dab, lipothymia, lipothymy, lollapaloosa,
   looker, lovely, lulu, mastery, model, moral victory, nirvana,
   nirvana principle, nothingness, oblivion, obliviousness, paragon,
   peach, picnic, pinup, pinup girl, pip, pippin, pushover, pussycat,
   quietus, raving beauty, reigning beauty, runaway victory,
   semiconsciousness, sensation, senselessness, settler, sex kitten,
   sleep, slick chick, smash, smash hit, sockdolager, stopper,
   stunner, stupor, subdual, subduing, success, sweetheart, swoon,
   syncope, the nuts, total victory, triumph, unconsciousness,
   victory, walkaway, walkover, whiz, win, winner, winning,
   winning streak