expression, godan verb (-ku)
play one's trump card; use one's secret weapon
Idiomatic expression meaning to finally bring out one's most powerful or decisive resource, often after holding it in reserve. Literally 'to draw the treasured sword of the family'.
交渉が難航したので、ついに伝家の宝刀を抜くことにした。
Negotiations were difficult, so we finally decided to play our trump card.
He brings out his secret weapon at the crucial moment.