unmentionable (adj.)youdaoicibaDictYouDict[unmentionable 词源字典]
1837, from un- (1) "not" + mentionable (adj.). Humorous use of unmentionables "trousers" is attested from 1823; from 1910 as "underwear," both on notion of "articles of dress not to be mentioned in polite circles."[unmentionable etymology, unmentionable origin, 英语词源]