1590s, "act of crushing, a violent collision or rushing together," from crush (v.). Meaning "thick crowd" is from 1806. Sense of "person one is infatuated with" is first recorded 1884, U.S. slang; to have a crush on (someone) is by 1903.
"grinding tooth, back-tooth," mid-14c., from Latin molaris dens "grinding tooth," from mola "millstone," from PIE root *mele- "to crush, grind." As an adjective, "grinding, crushing," as distinguished from "cutting" or "piercing," from 1620s. In Old English they were cweornteð "quern-teeth."
mid-15c., "instrument for crushing or pounding," from Middle Dutch braeke "flax brake," from breken "to break" (see break (v.)). The word was applied to many crushing implements, especially the tool for breaking up the woody part of flax to loosen the fibers. It also was applied to the ring through the nose of a draught ox. It was influenced in sense by Old French brac, a form of bras "an arm," thus the sense "a lever or handle," which was being used in English from late 14c., and "a bridle or curb" (early 15c.).
One or the other sense or a convergence of all of them yielded the main modern meaning "mechanical device for arresting the motion of a wheel," which is attested by 1772.
"a breaking asunder, a violent disruption," 1829, from Latinized form of Greek kataklasm "breakage," from kata "down" (see cata-) + klan,klaein "to break," which is perhaps from PIE *kla-, variant of root *kel- "to strike" (see holt), but more likely of uncertain origin [Beekes]. Related: Cataclastic, in geology, in reference to a structural character due to intense crushing, 1885.