The Collaborative International Dictionary of English v.0.48:

Haymaker \Hay"mak`er\ (h[=a]"m[=a]k`[~e]r), n.
   1. One who cuts and cures hay.
      [1913 Webster]

   2. A machine for curing hay in rainy weather.
      [1913 Webster]

   3. A forceful punch that results in someone being knocked
      down or knocked out; as, he delivered a haymaker to his
      opponent's jaw. [slang]
      [PJC]

WordNet (r) 3.0 (2006):

haymaker
    n 1: a farm machine that treats hay to cause more rapid and even
         drying [syn: haymaker, hay conditioner]
    2: a hard punch that renders the opponent unable to continue
       boxing [syn: haymaker, knockout punch, KO punch,
       Sunday punch]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "haymaker":
   Bauer, Long Melford, agriculturalist, agriculturist, agrologist,
   agronomist, backhand, backhander, backstroke, bolo punch, box,
   buffet, chop, clout, coffee-planter, collective farm worker,
   crofter, cropper, cultivator, dirt farmer, dry farmer,
   farm laborer, farmer, farmhand, gentleman farmer, granger, grower,
   harvester, harvestman, hook, husbandman, kibbutznik, kolkhoznik,
   kulak, muzhik, one-two, paste, peasant, peasant holder, picker,
   planter, plowboy, plowman, poke, punch, raiser, rancher, ranchman,
   reaper, round-arm blow, roundhouse, rustic, sharecropper,
   short-arm blow, sidewinder, smack, sock, sower, swing, tea-planter,
   tenant farmer, tiller, tree farmer, truck farmer, uppercut,
   yeoman