Synonyms for Specious:

adj.

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

n.

allegedly, anecdotal, apparently, doubtful, seemingly, speculative, supposed, unfounded, unsubstantiated. correct, fallacious, illogical, sophistic, true, unsound. counterfactual, untrue, untruthful, wrong, Truthless. plausible (noun)
apparent, beguiling, casuistic, credible, deceptive, fallacious, hypocritical, likely, misleading, presumable, presumptive, probable, sophistical, unsound, Colorable, apparently right, mingly just.
specious (noun)
false, gilded, insincere, invalid, meretricious, spurious.

Other synonyms:

counterfactual, sophistic. untruthful, Truthless. fallacious. unsound, wrong. irrational
illogical.
Other relevant words:
alleged, allegedly, anecdotal, apparent, apparently, beguiling, casuistic, counterfactual, credible, deceptive, doubtful, fallacious, false, gilded, hypocritical, illogical, insincere, invalid, likely, meretricious, misleading, presumable, presumptive, probable, seemingly, sophistic, sophistical, speculative, spurious, supposed, true, unfounded, unsound, unsubstantiated, untrue, untruthful, wrong, Truthless, Colorable, apparently right.