WordNet sense 1 (plausible but false):

specious

WordNet sense 2 (not in accordance with the fact or reality or actuality):

false

WordNet sense 3 (born out of wedlock):

misbegotten, bastardly, misbegot

WordNet sense 4 (of marriages and offspring; not recognized as lawful):

illegitimate

WordNet sense 5 (intended to deceive):

inauthentic, unauthentic

WordNet sense 6 (not genuine; imitating something superior):

counterfeit, imitative