Green’s Dictionary of Slang

(US) a vehicle manufactured by the General Motors Corporation.

[1887		N.Y. Semi-weekly Trib. 18 Mar. n.p.: Two jimmies loaded with coal [F&H]].
1895		C. Crowe Aus. Sl. Dict. 58: Jimmy, an iron bar cloven at one end.
1962		D. Dempsey ‘Lang. of Traffic Policemen’ in AS XXXVII:4 272: A G.M.C. (General Motors Corporation) truck is called a Jimmy.
1974		(con. late 1940s) E. Thompson Tattoo (1977) 480: A Jimmy, I think. Yeah, a Jimmy. Blue cab. Dusty. I didn’t see no name on it.
1993		J. Mowry Six Out Seven (1994) 66: Used to roll the big seven cross most of Texas, bucked snow through the Rockies, an left them Jimmys pantin for breath up the Grapevine out of L.A.