All of the other definitions are false or sexual, let me clear things up. A Haymaker is a punch that is similar to a hook, but instead of the elbo bent at around 100 degrees, the elbo is only slightly bent (maybe 160 degrees) and usually hits the side of the body or even head. Widely used by unexperienced fighters because it is usually the first and easiest strike one can execute. But they can also be powerful if done right but is almost never very accurate, deeming it impractical in most cases unless one really knows what he/she is doing; even though it is so widely used. Unexperienced fighters usually only hit with their dominant hand too.
-Bubba Joe: Hey fag! I fucked your mom last night!
-John Doe: Dude I don't like people talking about my mom like that, she died you son of a bitch!
-Bubba Joe: HAHA! I'm so macho!!!
-John Doe: *pushes Bubba Joe knocking him on his ass*
-Bubba Joe: *gets up and throws a bunch of haymakers and misses*
-John Doe: *Shovel hooks Bubba Joe in the face* HANDSTRIKESFACE!