Green’s Dictionary of Slang

1. a temporary worker; a substitute.

1918		‘Commander’ Clear the Decks! 174: Got the name for being a pinch hitter [...] he gained the reputation of being able to patch up his engines.
1935		J. Williams ‘The Whirling Hub’ in Afro-American (Baltimore, MD) 27 Apr. 15/1: Folks, the moniker on the by-line is that of a pinch-hitter for your regular columnst.
1951		C. Stoker Thicker ’n Thieves 364: Mayor Bowron’s move in installing Chief Worton as a pinch-hitter for Chief of Police Clemence B. Horrall was not prompted solely by a desire to restore police morale.

2. (drugs) one who is hired to inject an addict too ill to do it themselves.

1981

Spears Sl. and Jargon of Drugs and Drink (1986).