Up your Sleeve

Definition: A hidden advantage; some form of trickery lying in wait.
Example: 1) Be careful doing business with Tom -- he's always got something up his sleeve.
Etymology:

This is one of many slang phrases derived from gambling. When a card player has 'an ace up his sleeve' he is hiding an extra card (usually a powerful one like an ace) in his shirt sleeve, waiting to use it to win a hand. Now the phrase refers to any kind of hidden strength or advantage.

Bluesky