Adjectives

(s)spuriousostensibly valid, but not actually valid“a spurious argument”
(s)specious, spuriousplausible but false“a specious claim”, “spurious inferences”
(s)bastardly, misbegot, misbegotten, spuriousborn out of wedlock“the dominions of both rulers passed away to their spurious or doubtful offspring” (E.A.Freeman)
(s)inauthentic, unauthentic, spuriousfalse or fake; not what it appears to be“a spurious work of art”