Definition: The saying means that a person has a hidden or secret advantage.
Origins: In the 16th century, it was common for people to store things in their sleeves because clothes did not yet have pockets. The definition has changed slightly since then. The current meaning comes from gambling situations when dishonest players conceal ideal cards up their sleeves.
Ammer, Christine. The American Heritage Dictionary of Idioms. Boston: Houghton Mifflin Company, 1997, 103.