different between unmentionable vs villainous

English

un- +? mentionable

unmentionable (comparative more unmentionable, superlative most unmentionable)

unmentionable (plural unmentionables)

Something not to be discussed in polite society.
- 1995, Virginia Schroeder Burnham, William H. Hampton, Since Time Began (page 43)
  He was taught from childhood that the world does not condone homosexuality and that it is an unmentionable of decent people.
- 2014, Andrew Hodges, Alan Turing: The Enigma (page 100)
  Here, after all, was the circle round which Forster passed the manuscript of his novel Maurice which conveyed so much about being 'an unmentionable of the Oscar Wilde sort'.

From Old French vileneus

villainous (comparative more villainous, superlative most villainous)

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