Green’s Dictionary of Slang

jim n.5

[generic use of proper name; based on john n.2 (2c)]

a man who likes to watch prostitutes at work (or just a ‘dirty old man’ who frequents ‘adult’ bookshops, stripshows etc) but offers no actual sexual threat.

[UK]D. Powis Signs of Crime 189: Jims, or jimmies The ‘men in raincoats’ who watch prostitutes at work in an unhealthy but quite inactive manner.
[UK]J. Morton Lowspeak.