1
general::
Undercover or clandestine operations and deceitful stratagems in politics and espionage. For example, This campaign has been dominated by the dirty tricks of both sides. The adjective dirty here is used in the sense of "unethical" or "unfair." The term originally was applied to covert intelligence operations carried out by the Central Intelligence Agency, whose planning directorate was nicknamed "department of dirty tricks." It later was extended to underhanded activity intended to undermine political opponents and commercial rivals. [1940s]
American Heritage Idioms