1. 
[syn: incredibly, improbably, implausibly, unbelievably]
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):
implausibly
    adv 1: not easy to believe; "behind you the coastal hills plunge
           to the incredibly blue sea backed by the Turkish
           mountains" [syn: incredibly, improbably,
           implausibly, unbelievably] [ant: believably,
           credibly, plausibly, probably]