also side-kick, "companion or close associate," 1901, also side-kicker (1903, "O. Henry"), American English, of uncertain signification. Earlier terms were side-pal (1886), side-partner (1886).

pilot of the plane beside the lead aircraft in a formation, 1943 (earlier as a football position), from wing (n.) + man (n.). With figurative extensions, including the dating-sidekick one that was in use by 2006.