foul play

illegal activity; bad practices. The police investigating the death suspect foul play. Each student got an A on the test, and the teacher imagined it was the result of foul play.

Unfair or treacherous action, especially involving violence. For example, The police suspected he had met with foul play. This term originally was and still is applied to unfair conduct in a sport or game and was being used figuratively by the late 1500s. Shakespeare used it in The Tempest (1:2): "What foul play had we, that we came from thence?"