かくしだま

かくしだま【隠し球】

【野球】 a hidden-ball play; 〔交渉などでの最後の切り札〕 an ace up one's sleeve; a trump held back to be played when negotiations threaten to break down.

●こっちには隠し球があるんだ. この契約はうちが取ってみせる. I've got an ace up my sleeve and it's going to win that contract for us.

●相手側は交渉の席でとんでもない隠し球を出してきた. During the negotiations the other side came out with a surprise maneuver.

・交渉を有利に運ぶためには隠し球を持っていることだ. You should hold back a trump to assure that the negotiations progress advantageously.