up his sleeve

Up his sleeve is an idiom used when someone has something hidden or in secret but is ready to be used at any point he needs to. This is usually pertaining to a situation where there is some sort of competition or game and this person has a trick up his sleeve, which can be anything that might give him the upper hand, but he will wait to use it until the perfect opportunity presents itself. Usually he will not let this trick be known until it is time to be used.