Words related to rock
type of gait of a horse, between a trot and a gallop or canter, 1580s, from rack (v.) "move with a fast, lively gait" (1520s, implied in racking), which is of unknown origin; perhaps from French racquassure "racking of a horse in his pace," itself of unknown origin. Or perhaps a variant of rock (v.1).
1852, "a rocking chair," American English, agent noun from rock (v.1). Middle English had rokker, "nurse charged with rocking a cradle" (early 14c.). In sense of "one of the curved pieces of wood that makes a chair or cradle rock" it dates from 1787. Meaning "one who enjoys rock music" (opposed to mod (n.1)) is recorded from 1963, from rock (v.2).
Slang off (one's) rocker "crazy" is attested by 1897 according to OED; a widely reprinted 1903 newspaper column in U.S. identified it as British slang; the image is perhaps mechanical. To get (off) one's rocker seems to have been used earlier in U.S. baseball slang for "get busy, get active in a game" (1895) and does suggest the rocking-chair.
"full of rocks," late 15c., rokki, from rock (n.1) + -y (2). Earlier in Middle English as rochi (c. 1300), from French roche. The sense of "unsteady, unstable, tottering" is by 1737, from rock (v.1). The meaning "difficult, hard" is recorded from 1873, and may represent a bit of both.
The Rocky Mountains were so called by 1802, translating French Montagnes Rocheuses, first applied to the Canadian Rockies. "The name is not directly self-descriptive but is an approximate translation of the name of the former Native American people here known as the Assiniboin .... The mountains are in fact not noticeably rocky" [Room]. Bright notes that "These Indians were called /assiniipwaan/, lit. 'stone Sioux', by their Cree (Algonkian) neighbors".