What is the origin of an idiom have something up your sleeve?

It comes from magic and magicians. As most people know, magicians have very clever ways of concealing things for their tricks, and one of the most common hiding places for things like cards, handkerchiefs, and so on, is up the magician's usually-long sleeves.

Saying you have something up your sleeve means you still have some trick or idea you've yet to reveal and/or employ (such as, in gaming, some alternate strategy or tactic or a special play you've been concealing to this point).