The Collaborative International Dictionary of English v.0.48:

Knock down \Knock" down`\, n.
   1. To strike down; to fell; to prostrate by a blow or by
      blows; as, to knock down an assailant.
      [1913 Webster]

   2. To assign to a bidder at an auction, by a blow or knock of
      the auctioneer's hammer; to sell at an auction; as, the
      vase was knocked down at two thousand dollars.
      [1913 Webster +PJC]

   3. To take apart; to dissassemble; as, to knock down a rifle
      for cleaning.
      [PJC]

   4. To reduce or discount the price of; as, the dresses were
      knocked down to twenty dollars.
      [PJC]

   5. To earn (an income). "plumbers who knock down over a
      hundred thousand a year."
      [PJC]

WordNet (r) 3.0 (2006):

knock down
    v 1: cause to come or go down; "The policeman downed the heavily
         armed suspect"; "The mugger knocked down the old lady after
         she refused to hand over her wallet" [syn: down, knock
         down, cut down, push down, pull down]
    2: knock down with force; "He decked his opponent" [syn: deck,
       coldcock, dump, knock down, floor]
    3: shatter as if by explosion [syn: blast, knock down]