up one's sleeve
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.
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.