The Collaborative International Dictionary of English v.0.48:

Plausibility \Plau`si*bil"i*ty\, n. [Cf. F. plausibilit['e].]
   1. Something worthy of praise. [Obs.]
      [1913 Webster]

            Integrity, fidelity, and other gracious
            plausibilities.                       --E. Vaughan.
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   2. The quality of being plausible; speciousness.
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            To give any plausibility to a scheme. --De Quincey.
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   3. Anything plausible or specious. --R. Browning.
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WordNet (r) 3.0 (2006):

plausibility
    n 1: apparent validity [syn: plausibility, plausibleness]
         [ant: implausibility, implausibleness]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "plausibility":
   acceptability, admissibility, apparent soundness, believability,
   believableness, casuistry, circularity, color, common sense,
   conceivability, credibility, credit, disingenuousness, distortion,
   equivocalness, equivocation, evasive reasoning, fallaciousness,
   fallacy, insincerity, jesuitism, jesuitry, justifiability,
   justness, logic, logicality, logicalness, misapplication,
   mystification, obfuscation, obscurantism, oversubtlety, perversion,
   philosophism, plausibleness, rationality, rationalization, reason,
   reasonability, reasonableness, reliability, sense, sensibleness,
   sophism, sophistical reasoning, sophistication, sophistry,
   sound sense, soundness, special pleading, speciosity,
   specious reasoning, speciousness, subtlety, sweet reason,
   tenability, trustworthiness, vicious circle, vicious reasoning