From Greek 'autos' (self) — the word serves double duty as a clipping of 'automobile' (a Greek-Latin hybrid meaning 'self-moving') and as English's most productive prefix for self-referential concepts.
A motor car; also a prefix meaning 'self' or 'one's own'
Auto has a double life in English. As a standalone word meaning 'car,' it is a clipping of 'automobile,' coined in French in the 1870s-1880s from Greek 'autos' (αὐτός, self) and Latin 'mobilis' (movable) — a hybrid compound meaning 'self-moving.' The clipped form 'auto' appeared in English by 1899. As a prefix meaning 'self,' it has been productive in English since the 16th century, drawn directly from Greek
The word 'automobile' is a Greek-Latin hybrid — 'auto' (Greek for self) + 'mobile' (Latin for movable). Language purists in the 1890s objected to this cross-breeding and proposed all-Greek alternatives like 'autokineton' and all-Latin alternatives like 'ipsomobile.' None caught on. The hybrid won, and the abbreviated 'auto' stripped away even the Latin half.