The term 'Knock Out' is related to which sport?

In 1838, the London Prize Ring rules were codified. Later revised in 1853, they stipulated the following:

Fights occurred in a 24 feet (7.3 m)-square ring surrounded by ropes.

If a fighter were knocked down, he had to rise within 30 seconds under his own power to be allowed to continue.

Biting, headbutting and hitting below the belt were declared illegal.