Spurious

1. adjective

   born out of wedlock; - E.A.Freeman

   “the dominions of both rulers passed away to their spurious or doubtful offspring”

   Similar: bastardly, misbegot, misbegotten

2. adjective

   plausible but false

   “a specious claim”

   Similar: specious

3. adjective

   intended to deceive

   “a spurious work of art”

   Similar: inauthentic, unauthentic