Im*plau"si*bly

from 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]