Synonyms of fallacious

1 Syllable
mad, bad, wrong, off
2 Syllables
falser, falsest, baddest, untrue, phony, truthless, cruddy, way-off, floater, amiss, faulty, unreal, wronging, faultier, blue-sky, fishy, spurious, wronger, fluffed, wrongest, badder, unsound, specious
3 Syllables
deceitful, casuistic, incorrect, erroneous, mistaken, off-the-wall, sophistic, ill-founded, misleading, invalid, unbacked, phantasmal, unfounded, fictitious, phantasmic, beguiling, ungrounded, deceiving, deluding, fraudulent, reasonless, mendacious
4 Syllables
delusive, illusory, illogical, irrational, elusive, unreasoning, deceptive, barmecidal, cockamamie, imprecise, delusory, casuistical, misfigured, sophistical, unreasoned, elusory
5 Syllables
unreasonable, inaccurate, phantasmagoric, unretentive
6 Syllables
hallucinatory
7 Syllables
misrepresentative