A haymaker is a powerful punch or a knockout punch. Usually, a haymaker is a punch that involves a swinging arc, delivering a large amount of power at impact. This visual echos the swinging of the scythe in harvesting hay in the field. The term comes from the sport of boxing, the earliest known use of the word haymaker occurred in American newspapers at the very turn of the twentieth century. Generally, a haymaker is a boxing punch that is easy to see coming and is only employed when the boxer is confident he will knock his opponent out with the blow.
Credit: https://grammarist.com/usage/haymaker/
