Source : Webster's Revised Unabridged Dictionary (1913)

Knockdown \Knock"down`\, n.
   A felling by a knock, as of a combatant, or of an animal.

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

Knockdown \Knock"down`\, a.
   1. Of such force as to fell or overthrow; overwhelming; as, a
      knockdown blow.

   2. Designating a rivet end to be formed into a head by
      upsetting in fastening.

   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.

   4. Made or constructed so as to be capable of being knocked
      down or taken apart, as for transportation.

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.]

   2. A knocking down; a felling by a knock; a blow that
      overwhelms; also, a fist fight.

   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.

Source : WordNet®

knockdown
     adj : (furniture) easily assembled and dismantled; "I bought a
           knockdown chest at the do-it-yourself store"

knockdown
     n : a blow that knocks the opponent off his feet