Below the belt
An unfair, underhand tactic.
The London Prize Ring Rules were drafted by the boxer Jack Broughton in 1743. These included:
"That no person is to hit his Adversary when he is down, or seize him by the ham, the breeches, or any part below the waist a man on his knees to be reckoned down."
The rules were updated later and eventually superseded by the Marquis of Queensbury Rules in 1867, which form the basis of the rules for modern day boxing.
Oddly, although the rules were British, the term 'below the belt' appears first in the USA. Here's an early example from the New York Daily-Times, June 1853:
"... he will always respect that noble rule of pugilistic chivalry and 'never strike below the belt'."