Benchwarming – Benchwarmer – Warm the Bench (1888)

“If young Duffy keeps on he will come pretty near playing short for Chicago, with Williamson at third and Tommy Burns a bench warmer.” The Boston Daily Globe, March 20, 1888, p11

“Cleveland is the man Washington wants for third base, and he would like to go there, as he would then escape the disagreeable bench warming.” St. Louis Daily Globe-Democrat, May 24, 1888, p8

“Blair has earned most of his salary this season warming a bench, and before the game was three minutes old Mr. Sharsig was probably sorry he had not permitted the phenom to earn another day’s salary watching the game.” The Enquirer [Cincinnati], September 20, 1888, p2

Earlier examples of other senses:

Political sense
“He served as alderman in 1854 and 1855, and though he failed to distinguish himself in that position—being denominated in political slang a ‘bench-warmer,’ that is a man who keeps his seat and says nothing in public assemblies….” New York Times, September 17, 1874, p4 

Religious sense (said of people who sit in church but are not sincere in their commitment to religion)
“Oh, how I have been outraged by these bench warmers who come and keep out the poor sinner who would come in. We can put up with bench warmers for a time, but next week we may be ready to buy them a ticket to get out of town and make room for other people.” The Boston Daily Globe, January 19, 1887, p1

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1887. “Mr. Benchwarmer—Well, the base-ball season is nearly over. Mrs. Benchwarmer—Well, what of it?” (Philadelphia Call, quoted in Chicago Daily Tribune, Oct. 9). (Benchwarmer)

NOTE: (Ken Liss) The 1887 “Mr. Benchwarmer” article, though mentioning baseball, does not really seem to use the term in the modern sense. It seems more likely to be a reference to the fact that “Mr. Benchwarmer” is no longer getting exercise now that baseball season is over.

1892. “It’s about time he [Cap Anson] gave a younger man a chance to play on the team while he warmed the bench” (The Boston Globe, Apr. 10; Peter Morris). (Warm the Bench) 

1902. “Drill is as good for a recruit as Clarke is for a veteran, has hit for .281, and is excusable by reason of bench warming” (Paul W. Eaton, The Sporting Life, Aug. 30; Peter Morris). (Benchwarming)