The term "haymaker" originated around 1400 and referred to a person who made hay by swinging a scythe, a long staff with a curved blade used to cut through the grass and weeds that comprise hay. Around 1907, the word was used to refer to an incredibly strong punch, usually made with full force and the same swinging motion made when swinging a scythe.
Swinging a scythe through a field of grass took incredible strength, and the word is also thought to refer to the force with which a 15th century haymaker could hit someone.
In modern day, the haymaker is considered an impure punch in boxing as it takes a long time to wind up due to the force required to make the proper weight transfer. It is intended to knock an opponent out. A haymaker can also be a shoulder punch like that used in mixed martial arts from a grappling position. This punch leaves the attacker vulnerable to counter if the punch misses or is blocked.
A haymaker can be defended against with a grappling move, by rerouting the attacker's momentum, with a quick strike to the face or with a block that cuts off the punch's momentum by striking at the attacker's bicep.