1899 as shortened form of automobile (q.v.). Similar evolution yielded French, German auto.

word-forming element of Greek origin meaning "self, one's own, by oneself, of oneself" (and especially, from 1895, "automobile"), from Greek autos, reflexive pronoun, "self, same," which is of unknown origin. It also was a common word-forming element in ancient Greek, as in modern English, but very few of the old words have survived the interval.

In Greek, as a word-forming element, auto- had the sense of "self, one's own, of oneself ('independently'); of itself ('natural, native, not made'); just exactly; together with." Before a vowel, it became aut-; before an aspirate, auth-. In Greek it also was used as a prefix to proper names, as in automelinna "Melinna herself." The opposite prefix would be allo-.

"sentence passed by the Inquisition" (plural autos-da-fé), 1723, from Portuguese auto-da-fé "judicial sentence, act of the faith," especially the public burning of a heretic, from Latin actus de fide. The elements are auto "a play," in law, "an order, decree, sentence," from Latin actus (see act (v.)), de "from, of" (see de), fides "faith" (from PIE root *bheidh- "to trust, confide, persuade"). The Spanish form is auto-de-fe, but the Portuguese form took hold in English, perhaps through popular accounts of the executions following the earthquake of 1755.

"device that focuses automatically," by 1933 in photography, originally of enlargers; by 1942 of lenses, from auto- + focus (n.).

"German expressway," 1937, from German Autobahn (1930s), from auto "motor car, automobile" (short for automobil; see auto) + bahn "path, road," from Middle High German ban, bane "way, road," literally "strike" (as a swath cut through), from PIE *gwhen- "to strike, kill" (see bane).

also auto-immune, "arising from an abnormal immune response to a normal body part," 1952, from auto- + immune. Related: Autoimmunity, attested by 1903 as "immunity, natural or acquired, effected by the unaided powers of the organism, independent of external agencies." The modern sense of "immune responses of an organism against its own healthy cells and tissues" is from 1950s.

"car which contains in itself a motor and a source of power," 1895, from auto- + car.

Which is it to be? We observe that the London Times has lent the weight of its authority to the word "autocar," which it now prints without the significant inverted commas but with a hyphen, "auto-car." We believe that the vocable originated with a journal called the Hardwareman, which succeeded in obtaining the powerful support of the Engineer for its offspring. As for ourselves, being linguistic purists, we do not care for hybrid constructions—"auto" is Greek, while "car" is Latin and Celtic. At the same time, such clumsy phrases as "horseless carriages," "mechanical road carriages," and "self-propelled vehicles" are not meeting with general favour. Why not therefore adopt the philogically sound "motor-car," which could be run into a single word, "motorcar"? [The Electrical Engineer, Dec. 20, 1895]

Compare automobile.

"self-deification," 1610s, from auto- + -theism. The religion of one who mistakes his own inner voices for God's voice in him. Also used in a theological sense (1580s) for "the regarding of the second person of the Trinity as God entire." Related: Autotheist; autotheistic.

"self-generated," 1846, earlier autogeneal (1650s), from Greek autogenetos "self-born," from autos "self" (see auto-) + genetos "born," from genes "formation, creation" (from PIE root *gene- "give birth, beget"). The modern form and biological use of the word are said to be due to English paleontologist Richard Owen.

1640s, "one sprung from the soil he inhabits" (plural autochthones), from Latinized form of Greek autokhthon "aborigines, natives, primitive inhabitants," literally "sprung from the land itself," used of the Athenians and others who claimed descent from the Pelasgians, from autos "self" (see auto-) + khthōn "land, earth, soil" (from PIE root *dhghem- "earth").