IPA: /ˌʌnəˈθɛntɪk/
KK: /ʌnɔˈθɛntɪk/
Not genuine or real; lacking authenticity.
The painting was deemed unauthentic and not worth much.
Unauthentic → It is formed from "un-" (meaning not) and "authentic" (from Greek "authentikos", meaning genuine or real). The word "unauthentic" means not genuine or not real.
Think of 'not' ('un-') and something that is 'genuine' ('authentic') — that's why unauthentic means not genuine.