'unauthentic' originates from the prefix 'un-' meaning 'not' and the word 'authentic', which comes from the Greek word 'authentikos', meaning 'original' or 'genuine'.

'authentikos' transformed into the Latin word 'authenticus', and eventually became the modern English word 'authentic', with 'unauthentic' forming by adding the prefix 'un-'.

Initially, 'authentic' meant 'original' or 'genuine', and 'unauthentic' has consistently meant 'not genuine' or 'fake'.