Green’s Dictionary of Slang

a euph. for a variety of negative personal descriptions.

1933		J. Gibbons Truth About the Legion 64: ‘We don’t want’, he said, ‘all the Condemned Books on the Sanguinary Legion to be written by the Unmentionables who have Run Away from It’.
1953		T. Runyon In For Life 123: The double-censored old unmentionable was all but crippled.