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.
      --Chaucer.
      [1913 Webster]

   2. That which is untrue; a false assertion; a falsehood; a
      lie; also, an act of treachery or disloyalty. --Shak.
      [1913 Webster]

   Syn: Lie; falsehood. See Lie.
        [1913 Webster]

WordNet (r) 3.0 (2006):

untruth
    n 1: a false statement [syn: falsehood, falsity, untruth]
         [ant: true statement, truth]

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