Why is the ninth month called September?
word-forming element meaning "self, one's own, by oneself," from Greek auto- "self, one's own," combining form of autos "self, same," of unknown origin. Before a vowel, aut-; before an aspirate, auth-. In Greek also used as a prefix to proper names, e.g. automelinna "Melinna herself." The opposite prefix would be allo-.
auto- or aut-
Self; same: autogamy.
A prefix meaning "oneself," as in autoimmune, producing antibodies or immunity against oneself. It also means "by itself, automatic," as in autonomic, governing by itself.