IPA: /ˌɪnəˈθɛntɪk/
KK: /ɪnɔˈθɛntɪk/
Not real or true; lacking authenticity or genuineness.
The painting was found to be inauthentic, as it was a copy rather than an original.
Inauthentic → It is formed from "in-" (meaning not) and "authenticus" (from Greek "authentikos", meaning genuine or real). The word "inauthentic" means not genuine or not real.
Think of 'not' ('in-') and 'genuine' ('authenticus') to remember that inauthentic means something that is not genuine.