The story behind this term is quite unusual and far from what one might guess. The term “sidekick” comes from a pickpocket’s jargon, and it means the front pocket in a pair of men’s trousers. It is considered the hardest pocket to pick, since it is always in motion and because the man’s hand is constantly swinging back and forth past it. It is, therefore, the safest and most dependable place for a man to carry his wallet. Somehow this idea of dependability came to be associated with a trusty companion, and the same term was used to describe him.

From: More Ever Wonder Why?
by: Douglas B. Smith