c. 1500, "outer side, the exterior part or surface of a thing," from out- + side (n.). Meaning "the part or place that lies without or beyond an enclosure or barrier" is from 1610s. In isolated regions of the globe it tends to mean "the world of civilization and settlement" (1827); in prison (and army) slang, "the world outside prison (or the army)," by 1903.
The adjective is attested from 1630s, "being on the outside; to the outer surface or boundary;" as "situated or operating outside (the house, the system, etc.) by 1841; as "not directly concerned or interested" by 1881. As an adverb from 1813 "on the outside, on or to the exterior;" as a preposition from 1826. Colloquial phrase outside of "with exception of" is from 1859. Outside chance "very unlikely chance" is by 1845, originally in horse racing (see outsider).