haymaker

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A powerful blow with the fist.

from The Century Dictionary.

• noun One who cuts and dries grass for use as fodder; specifically, in England, one who follows the mowers and tosses the grass over to dry it.
• noun An apparatus for drying and curing hay.
• noun plural A kind of country-dance. Also called haymakers' jig.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who cuts and cures hay.
• noun A machine for curing hay in rainy weather.
• noun slang A forceful punch that results in someone being knocked down or knocked out.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun agriculture A person or machine which harvests or prepares tall grass for use as animal fodder.
• noun informal A particularly powerful punch, especially one which knocks down an opponent, thrown like a scythe chop for cutting hay, as agricultural haymakers used to have strong arms.
• noun figuratively, by extension Any decisive blow, shock, or forceful action.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a hard punch that renders the opponent unable to continue boxing
• noun a farm machine that treats hay to cause more rapid and even drying