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.