From Greek 'authentēs' (one who acts independently), from 'autos' (self) — an authentic thing is 'self-accomplished,' made by its true author.
Of undisputed origin; genuine; truly what something is said to be.
From Old French "autentique" (authentic, canonical), from Latin "authenticus" (authoritative, genuine), from Greek "authentikos" (original, primary, genuine), from "authentēs" (one who acts on his own authority, a doer, a master). The Greek "authentēs" is composed of "auto-" (self) and a second element "-hentēs" related to the verb "anuein" or "hanuein" (to accomplish). The prefix "auto-" derives from Proto-Indo-European *h₂ew- (self, apart), which also produced Latin "aut" (or — originally "on the other hand
The Greek word 'authentēs' had a darker original meaning: 'one who acts with their own hand' could mean a 'doer' but also a 'murderer' — someone who kills with their own hand rather than through an agent. The word softened from 'killer' to 'master' to 'author/originator,' and 'authentikos' came to mean 'done by the original hand.' The same 'auto-' (self) prefix appears in 'automobile' (self-moving), 'autobiography' (self-life-writing), and 'autopsy' (seeing for oneself).