card up one's sleeve
Also, ace up one's sleeve. A hidden or secret advantage or resource, as in Before we make a decision, let's see if management has another card up its sleeve, or You can count on John to have an ace up his sleeve. The practice of storing something in one's sleeve dates from the 16th century, when clothes rarely had pockets. The current term comes from gambling, where a dishonest player might so conceal an ace or other winning card. [Mid-1800s]