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Webster 1828 Edition

Adj.

s as z. [in and plausible.]

Not specious; not wearing the appearance of truth or credibility, and not likely to be believed; as an implausible harangue.

implausible ‎(comparative more implausible, superlative most implausible)

Not plausible; unlikely; dubious.
- 2008, February 17, Mark Liberman, "More on Harper", Language Log,
  Harper finds the idea that Latin developed into the modern Romance languages too implausible to believe.

unlikely