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]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "knock down":
   auction, auction off, auctioneer, bang, bash, bat, beat down,
   belittle, belt, biff, blow down, blow over, bonk, bowl down,
   bowl over, break down, bring down, bring into discredit, bring low,
   bulldog, bulldoze, burn down, cast down, chop down, clap, clip,
   clobber, clout, clump, coldcock, crack, crush, cry down, cut,
   cut down, damp, dampen, dampen the spirits, darken, dash,
   dash down, deal, deal a blow, debase, deck, decry, degrade, deject,
   demoralize, depreciate, depress, derogate from, detract from,
   disapprove of, discourage, discredit, disgrace, dishearten,
   disparage, dispirit, down, drop, fell, fetch, fetch a blow,
   fetch down, flatten, floor, ground, hew down, hit, hit a clip,
   hold in contempt, jab, knock, knock cold, knock out, knock over,
   lay, lay level, lay low, lay out, let have it, level, lower,
   lower the spirits, make little of, minimize, mow down, oppress,
   overcome, paste, plunk, poke, precipitate, press down, prostrate,
   psych out, pull down, punch, put down, rase, raze,
   reflect discredit upon, run down, sadden, sell by auction,
   send headlong, shake, sink, slam, slight, slog, slug, smack, smash,
   smite, snap, soak, sock, speak ill of, spread-eagle, steamroller,
   strike, strike at, submit to indignity, supinate, swat, swipe,
   take down, tear down, throw, throw down, thump, thwack, topple,
   trip, tumble, unbrace, undo, unman, unnerve, unstring, upset,
   wallop, weigh heavy upon, weigh upon, whack, whack down, wham,
   whop, win, yerk