Old English grindan "to rub together, grate, scrape," forgrindan "destroy by crushing" (class III strong verb; past tense grand, past participle grunden), from Proto-Germanic *grindanan (cf. Dutch grenden), related to ground, from PIE *ghrendh- "to grind" (cf. Latin frendere "to gnash the teeth," Greek khondros "corn, grain," Lithuanian grendu "to scrape, scratch"). Meaning "to make smooth or sharp by friction" is from c.1300. Most other Germanic languages use a verb cognate with Latin molere (cf. Dutch malen, Old Norse mala, German mahlen).
late 12c., "gnashing the teeth," from grind (v.). The sense "steady, hard work" first recorded 1851 in college student slang (but cf. gerund-grinder, 1710); the meaning "hard-working student" is American English slang from 1864.
bump and grind, if you can't find 'em
(Ex. 32:20; Deut. 9:21; Judg. 16:21), to crush small (Heb. tahan); to oppress the poor (Isa. 3:5). The hand-mill was early used by the Hebrews (Num. 11:8). It consisted of two stones, the upper (Deut. 24:6; 2 Sam. 11:21) being movable and slightly concave, the lower being stationary. The grinders mentioned Eccl. 12:3 are the teeth. (See MILL.)