Green’s Dictionary of Slang

a masturbator; also fig. use, a loser.

1975		A. Goldman Lenny Bruce 123: Arty dance acts [...] got to be too goddamned much for the strokers in the third row with the newspapers over their joints.
1997		(con. 1960s) A. Frewin London Blues 68: Charlie describes these books as strokers’ manuals, wankers’ digests and ‘handbooks for geezers who want the solution in their fists’.
1997		Eble Campus Sl. Apr. 8: stroker – male who tries hard to be successful with women but fails.