something up your sleeve来表示“秘而不宣的计划；锦囊妙计”。

【例句】
If I had stopped to think, I’d have known you had a lone hand up your sleeve.
假如我停下来想想，就能明白的你锦囊妙计了。

If you attend a social function, it is always advisable to have something up your sleeve, in case you are called upon to speak at short notice.
如果你参加社交活动，最好还是备一些东西，以防在接到通知后很快被要求发言。