A “cowboy Cadillac” is a pickup truck or other open-backed vehicle. The term was made popular by a bumper sticker in 1975.

See also Texas Cadillac


(Historical Dictionary of American Slang)
cowboy Cadillac n. S.W. a pickup truck or open-backed automobile. Joc.
1976 Lieberman & Rhodes CB Handbook 126: Cowboy Cadillac—An El Camino or Ford Ranchero (open-backed cars).


31 August 1975, Washington Post, pg. C5:
This year, the flag decals and the “Love It or Leave It” bumper stickers are few. “Cowboy Cadillac” on a pickup truck, or “Crime Doesn’t Pay—Neither Does Ranching” are typical of the new bumper crop—self-deprecating humor instead of hate.


14 October 1976, Oakland (CA) Tribune, pg. 49:
A pickup truck in Moraga with a sign on the back: “Cowboy Cadillac.”


8 December 1977, Galveston (TX) Daily News, pg. 6D ad:
Come in and take advantage of these unheard of discounts on our Fine Collection of Cowboy Cadillacs.