[up one's sleeve] or [in one's sleeve] {adv. phr.}
1. Hidden in the sleeve of one's shirt or coat and ready for secret or wrongful use.
* /The crooked gambler hid aces up his sleeve during the card game so that he would win./
2. {informal} Kept secretly ready for the right time or for a time when needed.
* /Jimmy knew that his father had some trick up his sleeve because he was smiling to himself during the checker game./
Compare: CARD UP ONE'S SLEEVE. 3. See: LAUGH UP ONE'S SLEEVE.