implausible

From im- +‎ plausible.

implausible (comparative more implausible, superlative most implausible)

1. Not plausible; unlikely; dubious.

   Antonym: plausible

   • 2008 February 17, Mark Liberman, “More on Harper”, in Language Log:

     Harper finds the idea that Latin developed into the modern Romance languages too implausible to believe.

• (not plausible): unplausible