Green’s Dictionary of Slang

1. an entertainment establishment that uses a front-man to solicit customers and runs continuous performances.

1927		H.L. Mencken Amer. Mercury XII 194/2: It must present ‘Naked Truth’ shows, and it must go all the way down the line to the grind joint.
1937		New Yorker 7 Aug. 19: The ‘grind joints,’ as they are called in the jargon of the [auction] racket, know...that they can trade only with strangers.
1990		D.F. Friedman Youth in Babylon 353: A twenty-four-hour triple-feature movie house is a good example of a ‘grind joint.’.

2. a third-rate casino.

1978		Skolnick House of Cards 40: Downtown casinos [...] are called in the trade ‘grind joints,’ a term suggesting a larger number of less affluent bettors [...] and heavy slot machine action [HDAS].
2000		P. Earley Super Casino 101: [C]asino owners were stunned to see how rich Bennett and Pennington had become operating grind joints—so called because profits were ground out a penny at a time.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 76: Owner of a bottom-rung or ‘Grind Joint’ casino.