"full of rocks," c. 1400, from rock (n.1) + -y (2); "unsteady," 1737, from rock (v.1). 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".