dirty trick

An action used for one's benefit that is unethical or dishonest. Every election cycle, the competing candidates always accuse each other of dirty tricks. I really wish you had been honest with me from the start about causing damage to my car. Blaming the neighbor was a real dirty trick.

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]

Covert, deceitful, and damaging policies and actions. The term dates from the later 1600s and was long used simply to describe a nasty action, as in “When no one was looking Jim grabbed the last decent sleeping bag; that was a dirty trick.” However, it acquired a new meaning during the 1960s when it was applied to the activities of the Central Intelligence Agency (CIA), which was even called “Department of Dirty Tricks” for some of its propaganda and secret interventions in other countries’ affairs. In the 1970s it began to be applied to the unethical and even illegal tactics of political campaigns, such as smearing an opponent with false accusations, illicitly using campaign funds, and the like.