expression, godan verb (-u)
play one's trump card; use one's last resort
Idiomatic expression for bringing out one's strongest or final hidden resource, often when other options have failed.
交渉が難航したので、ついに奥の手を使うことにした。
Negotiations were difficult, so I finally decided to play my trump card.
切り札 is a literal trump card in card games and also a figurative last resort, but 奥の手を使う emphasizes the action of deploying a hidden or ultimate strategy.
The expression combines 奥 (inner, back) and 手 (hand, method), suggesting a technique kept in reserve. The exact historical origin is uncertain, but it likely derives from martial arts or strategic contexts where a hidden technique is revealed only when necessary.