Green’s Dictionary of Slang

1. an assistant, a partner, an accomplice.

1905		H. Green Actors’ Boarding House (1906) 85: The Red Swede, who was a yegg man [...] sat over a pint of champagne with Dopey Polly, from Chinatown, and his side kick, the Runt.
1913		B.T. Harvey ‘Word-List From The Northwest’ in DN IV:i 28: side-kick, n. A pardner.
1922		Randall Parrish Case and the Girl 335: He’s Hogan’s side-kick [DA].
1927		W. Edge Main Stem 165: Hello, there. Oh, yes, Slim’s sidekick. How’s the world ben treatin’ ye?
1932		(con. 1929) R.E. Burns I Am a Fugitive 175: ‘Indiana’ was Nub’s pal and sidekick.
1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 336: sidekick—a close friend.
1935		(ref. to 1910) D. Maurer ‘Lingo of the Good People’ AS X:1 20/2: Side-kick [formerly] one’s pal […].
1942		R. Chandler High Window 103: Your sidekick sneaks out to the kitchen and gives me advice behind your back.
1949		R. Chandler Little Sister 104: He wanted protection or he wanted a sidekick.
1953		L. Uris Battle Cry (1964) 397: Captain Shapiro and his sidekick, Gunner McQuade.
1958		C. Himes Real Cool Killers (1969) 73: Where’s your sidekick, Coffin Ed.
1965		C. Brown Manchild in the Promised Land (1969) 23: None of my sidekicks was over twelve years of age.
1974		(con. 1960s) R. Price Wanderers 202: Two sidekicks stood on each side of him, like identical bookends.
1976		R. Sabbag Snowblind (1978) 82: That evening Blackie’s sidekick and dope curator, Fernando, rolled one of his special joints – an entire issue of Time magazine rolled around almost half an ounce of Colombian chiba-chiba.
1987		B. Chatwin Songlines 44: I’m the Grand Duke’s sidekick.
1990		S. Morgan Homeboy 14: Cops were looking for you last night. Had warrants for you and your sidekick.
1999		K. Sampson Powder 65: What he was doing with great competence was playing the role of the sidekick.
2000		Observer Screen 20 Feb. 18: It is sidekick Ray who delivers the winner.
2010		K. Richards Life 297: He had a bunch of sidekicks, this team of cowboys who we decided were safer on the payroll than off.
2010		L. Redhead Thrill City [ebook] ‘You seem to have forgotten that I’m your sidekick.’ ‘You’re not a sidekick, you’re my best friend’.
2018		G. Gilmore Class Act [ebook] ‘He’s always saving you [...] The perfect sidekick, really’.

2. in fig. use; something on which one depends; a regular pleasure.

1946

Mezzrow & Wolfe Really the Blues 87: Bull Durham was his sidekick before, but he couldn’t see nothing but corona-coronas now.