up one's sleeve, to have something

To have a hidden surprise or resource in reserve. The idea of keeping something up one’s sleeve dates at least from the sixteenth century, when clothes generally had no pockets and sleeves were a usual storage place. “He had contrary Edicts from the King in his sleeve,” wrote F. de L’Isle (Legendarie, 1577), and his meaning was probably both literal and figurative. In the nineteenth century a dishonest card shark might have an ace up his sleeve, an expression that came to be used figuratively in the same way.