The Collaborative International Dictionary of English v.0.48:

Knockdown \Knock"down`\, a.
   1. Of force sufficient to fell or completely overthrow; as, a
      knockdown blow; a knockdown argument..
      [Webster 1913 Suppl.]

   2. Designating a rivet end to be formed into a head by
      upsetting in fastening.
      [Webster 1913 Suppl.]

   3. Of or pertaining to the act of knocking down at an
      auction; specif., designating the price below which an
      article will not be disposed by the auctioneer.
      [Webster 1913 Suppl.]

   4. Made or constructed so as to be capable of being knocked
      down or taken apart, as for transportation.
      [Webster 1913 Suppl.] knock-down-and-drag-out

The Collaborative International Dictionary of English v.0.48:

Knockdown \Knock"down`\, n.
   1. That which knocks one down; something that overpowers or
      overwhelms, as strong liquor; specif., a kind of ale or
      beer that is very strong. [Slang.]
      [Webster 1913 Suppl.]

   2. A knocking down; a felling by a knock, as of a combatant,
      or of an animal; a blow that overwhelms; also, a fist
      fight.
      [Webster 1913 Suppl.]

   3. Something that knocks down, or takes apart, for packing or
      removal, as a piece of furniture; also, state of being
      knocked down, or taken apart.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

knock-down
    adj 1: strong enough to knock down or overwhelm; "a knock-down
           blow" [syn: knock-down(a), powerful]