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

WordNet (r) 3.0 (2006):

knockout
    adj 1: very strong or vigorous; "strong winds"; "a hard left to
           the chin"; "a knockout punch"; "a severe blow" [syn:
           hard, knockout, severe]
    n 1: a very attractive or seductive looking woman [syn:
         smasher, stunner, knockout, beauty, ravisher,
         sweetheart, peach, lulu, looker, mantrap, dish]
    2: a blow that renders the opponent unconscious [syn:
       knockout, KO, kayo]