Late 16th century; earliest use found in Richard Bernard (bap. 1568, d. 1641), Church of England clergyman and religious writer. From plausible + -ness.
Definition of plausibleness in US English:
plausibleness
nounˈplɔːzɪblnəs
The quality of being plausible; plausibility.
Origin
Late 16th century; earliest use found in Richard Bernard (bap. 1568, d. 1641), Church of England clergyman and religious writer. From plausible + -ness.