grind store noun an illegal gambling operation where players are cheated as a matter of course US
— Gene Sorrows, AllAboutCarnivals, p. 18, 1985: “Terminology”
But it was easy enough to find out that he ran a flat joint, also called a flat store or sometimes a grind store or simply a flat. — Nathaniel Knaebel, StepRightUp, p. 52, 2004