American pickpockets once called the side pants pockets side-kicks. These are the hardest pockets to pick because they are closest to the hands of a victim and are constantly moving with the motions of the legs. Therefore, any man wise to the ways of pickpockets kept his wallet in his trusty side pocket, or side-kick.

Side-kicker thus became a slang word for a faithful buddy, a partner who is always at one's side. O. Henry First recorded the term in one of his stories in 1904 and about ten years later side-kicker was shortened to sidekick.