1769, "act of fitting out (a ship, etc.) for an expedition," from out- + fit (v.). Sense of "articles and equipment required for an expedition" is attested from 1787, American English, hence the extended senses; meaning "a person's clothes" is first recorded 1852; sense of "group of people" is from 1883.
"to provide or supply, to fit out," 1840, from outfit (n.). Related: Outfitted; outfitting; outfitter.