Definition of "unmentionable"

He was taught from childhood that the world does not condone homosexuality and that it is an unmentionable of decent people.

1995, Virginia Schroeder Burnham, William H. Hampton, Since Time Began, page 43

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'.

2014, Andrew Hodges, Alan Turing: The Enigma, page 100