The Collaborative International Dictionary of English v.0.48:
Untruth \Un*truth"\, n.
1. The quality of being untrue; contrariety to truth; want of
veracity; also, treachery; faithlessness; disloyalty.
2. That which is untrue; a false assertion; a falsehood; a
lie; also, an act of treachery or disloyalty. --Shak.
Syn: Lie; falsehood. See Lie.
Moby Thesaurus II by Grady Ward, 1.0:
68 Moby Thesaurus words for "untruth":
aberrancy, aberration, blague, canard, cock-and-bull story,
defectiveness, delusion, deviancy, distortion, errancy,
erroneousness, error, exaggeration, fairy tale, fallaciousness,
fallacy, falsehood, falseness, falsity, farfetched story, farrago,
fault, faultiness, fib, fiction, fish story, flam, flaw,
flawedness, flimflam, ghost story, half-truth, hamartia, heresy,
heterodoxy, illusion, inveracity, legal fiction, lie,
little white lie, mendacity, misapplication, misconstruction,
misdoing, misfeasance, misinterpretation, misjudgment,
misrepresentation, peccancy, perversion, pious fiction,
prevarication, self-contradiction, sin, sinfulness,
slight stretching, story, tale, tall story, tall tale, taradiddle,
trumped-up story, truthlessness, unorthodoxy, untrueness,
untruthfulness, white lie, yarn