Kept secretly ready for the right time or for a time when needed.
Козырь в рукаве, запасной вариант.
If that medicine doesn’t work, I’ve got some other ways of treating you up my sleeve.
Если это лекарство не сработает, у меня есть еще кое-что в запасе.
Jimmy knew that his father had some trick up his sleeve because he was smiling to himself during the checker game.