Tā yīzhí bǎoliú zhe zhè gè mìmì, zuòwéi rìhòu de dǐpái.
He kept this secret all along as a card to play later.
Tips
usage
底牌 comes from card games where it refers to face-down cards. Figuratively, it means confidential leverage or a hidden advantage. The key phrase is 亮底牌 ('to reveal one's hand / show one's cards').
