Douglas Harper's Etymology Dictionary
bench-warmer
1892, baseball slang; see bench.\nThe days for "bench-warmers" with salaries are also past. ["New York Sporting News," Jan. 9, 1892]\nOld English had bencsittend "one who sits on a bench."
Wiktionary
bench-warmer
n. (context sports colloquial English) A player who rarely or never gets to play in the games or matches, and is most often a substitute.
Usage examples of "bench-warmer".
Suddenly there were people all around her, assistants, trainers, verrul grooms, second-string bench-warmers, gofers, and hangers-on.