A haymaker is usually described as you throwing the hardest punch you can. Usually when you're in a street fight with someone, you don't care about finesse, only about landing a 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.