A haymaker is usually described as you throwing the hardest
punch you can. When you're in a
street fight with someone, you don't care about
finesse, only about landing the haymaker.
An example of a haymaker is from any
John Wayne film. He'll bring his arm way back behind him and then bring it forward with all of his body weight into his opponent's
face.
POW!
It has been theorized the name come from making someone '
hit the hay' or go to
sleep.