Where does the term Haymaker Punch come from?

Answer 1

The term "haymaker" came into being because the path of the punch mimics the appearance of a scythe which was often used to manually cut hay. The scythe was used to chop it down before the hay was formed into bales.

Answer 2

it is a punch intending to knock someone out or put them to sleep or hit the hay, a haymaker punch makes them hit the hay or be knocked unconscious