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]