An inauthentic person is fake or insincere, and an inauthentic thing isn't what it's said to be. That gold Rolex watch you bought for five dollars? It's probably inauthentic.
Anything that pretends to be something it's not is inauthentic, whether it's a "designer" handbag or a person who's assuming a false identity. The adjective inauthentic is made up of the prefix in-, "not" or "opposite of," and authentic, "genuine" or "accurate." The Greek root is authentikos, "original, genuine, or principal."