Synonyms for Specious:

adj.

all (adjective)
spurious.
dishonest (adjective)
untrue, deceptive.
false (adjective)
untruthful, sophistical, untrue, casuistic.
misleading (adjective)
casuistic, apparently right, false, beguiling, untrue, illogical, credible, unsound, probable, wrong, presumptive, spurious, presumable, seeming, sophistic, sophistical, Colorable, fallacious, ostensible, likely, apparent, deceptive.
ostensible (adjective)
ostensible, seeming, alleged, pretended.

n.

doubtful, supposed, seemingly, unfounded, allegedly, unsubstantiated, apparently, speculative, anecdotal. counterfactual, untrue, untruthful, Truthless, wrong. specious (noun)
invalid, spurious, meretricious, false, gilded, insincere.

Other synonyms:

sophistic, counterfactual. Truthless, untruthful. fallacious. wrong, unsound. irrational
illogical.