in one's back pocket字面意思在某人裤子后口袋里，引申为囊中之物；绝密一招。

【例句】
Do you not think we'd be easy meat for him?
你没想过我们会变成他的囊中之物吗？

Keep that recording of your boss in your back pocket—you'll need it later if he denies what he said.
把你老板的录音收好，哪天他抵赖你就能用上了。