noun
forbidden move; prohibited move; foul
Used in sumo, shogi, go, and other competitive games for a move that is against the rules. Also used figuratively for an unfair or unacceptable action in other contexts.
相撲で禁じ手を使うと反則負けになる。
Using a forbidden move in sumo results in a loss by foul.
その政治家のやり方は禁じ手だと言われている。
That politician's methods are said to be a prohibited move.
反則 is a general term for a foul or rule violation in sports and games, while 禁じ手 specifically refers to a prohibited move or technique.
Compound of 禁じ (continuative form of 禁じる 'to forbid') + 手 ('move, technique').