From side + kick, which in the late 19th and early 20th century was a slang term for the front side pocket of a pair of trousers, was known as the pocket safest from theft. Thus, by analogy, a "side-kick" was a person's closest companion.
sidekick (plural sidekicks)
- (informal) An assistant to another person, especially to a superior or more important person.
- 2012 May 24, Nathan Rabin, “Film: Reviews: Men In Black 3”, in The Onion AV Club:
- In the abstract, Stuhlbarg’s twinkly-eyed sidekick suggests Joe Pesci in Lethal Weapon 2 by way of late-period Robin Williams with an alien twist, but Stuhlbarg makes a character that easily could have come across as precious into a surprisingly palatable, even charming man.