foul play

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Intentionally violent or injurious behavior, especially that suggestive of a criminal act.
• noun Inappropriate, unethical, or unlawful conduct, especially to disadvantage an opponent.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Unfair, unethical, or criminal behavior, especially of a malicious or violent nature.
• noun Any violation of rules.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun unfair or dishonest behavior (especially involving violence)