Green’s Dictionary of Slang

an assistant, a partner, an accomplice.

1896		F. Hutcheson Barkeep Stories 32: ‘I [...] tells dat dem two gen’lmen, pointin’ out me fren’ de politician an’ his side-kicker, de hobo, had just come in’.
1904		Sydney Sportsman (Surry Hills, NSW) 13 Apr. 3/6: She can’t for the life of her understand what Mrs. Noah was about to allow her old side-kicker to take mice into the Ark.
1908		‘O. Henry’ ‘The Octopus Marooned’ in Gentle Grafter (1915) 16: The whole town [...] is up in Sperry’s wool warehouse listening to your side-kicker make a speech.
1910		‘O. Henry’ ‘A Night in New Arabia’ in Strictly Business (1915) 219: She was Celia’s sidekicker and chum.
1916		J. Lait ‘Canada Kid’ in Beef, Iron and Wine (1917) 154: A new lad is brung in, an’ who could it be but my ol’ sidekicker an’ childhood neighbour, Chiggers.
1926		J. Black You Can’t Win (2000) 155: Doubtful about being able to do anything worth while by myself, I cast about for a ‘sidekicker’.
1927		(con. WW1) P. MacDonald Patrol 71: A kid called Brown . . . he was a sidekicker o’ mine’.
1931		O. Strange Law O’ The Lariat 157: That young side-kicker o’ Severn’s called him ‘Don’ that night.
1945		in A.J. Liebling Honest Rainmaker (1991) 85: ‘Buddy’ Prior [...] side-kicker of Wingie of Philly.