dirty trick; underhanded move
Refers to a cunning, malicious, or unfair tactic, often in competition or conflict. The kanji suggest a 'poisonous hand'.
彼は選挙で毒手を使ったと非難された。
He was accused of using dirty tricks in the election.
Compound of 毒 (poison) and 手 (hand), literally 'poison hand', figuratively a harmful or underhanded method. The exact historical derivation is uncertain, but the meaning is transparent from the kanji.