The Collaborative International Dictionary of English v.0.48:

Implausible \Im*plau"si*ble\, a. [Pref. im- not + plausible: cf.
   F. implausible.]
   Not plausible; not wearing the appearance of truth or
   credibility, and not likely to be believed. "Implausible
   harangues." --Swift. -- Im*plau"si*ble*ness, n. --
   Im*plau"si*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

implausible
    adj 1: having a quality that provokes disbelief; "gave the
           teacher an implausible excuse" [ant: plausible]
    2: highly imaginative but unlikely; "a farfetched excuse"; "an
       implausible explanation" [syn: farfetched, implausible]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "implausible":
   a bit thick, a bit thin, absurd, beyond belief, debatable,
   doubtable, doubtful, dubious, dubitable, fishy, flimsy,
   hard of belief, hard to believe, hardly possible, improbable,
   inconceivable, incredible, more than doubtful,
   not deserving belief, open to doubt, open to suspicion,
   passing belief, preposterous, problematic, puzzling, questionable,
   ridiculous, staggering belief, suspect, suspicious, tall, thick,
   thin, unbelievable, unconvincing, unearthly, ungodly, unimaginable,
   unlikely, unpromising, unreasonable, unsubstantial, unthinkable,
   unworthy of belief, weak