Dishonest or treacherous behaviour; also violent conduct.
'Foul play' is a 16th century idiom. Nowadays we often use this phrase in regard to 'fouls' that are committed in sports, that is, actions which are outside the particular sports' rules. This is itself quite an old usage; for example, from boxing - The Sporting Magazine, 1797:
"His antagonist having struck him two foul blows."
... and from billiards - The Field, January 1882:
"Thus, at billiards, if a player makes a foul stroke and scores, his adversary has the option of not enforcing the penalty."
These were preceded by Shakespeare's use, and probably his coinage, of the phrase in a non-sporting context, simply to mean 'unfair behaviour'; for example, Love's Labours Lost, 1588:
Honest plain words best pierce the ear of grief;
And by these badges understand the king.
For your fair sakes have we neglected time,
Play'd foul play with our oaths.
Shakespeare used this phrase in several plays, including Henry IV, The Tempest and Pericles, Prince of Tyre.
See also - fair play.
See other phrases and sayings from Shakespeare.