Victorian Era euphemism, specifically the category of euphemism known as indirection (compare privates, behind, sleep together). First intended meaning was "trousers", attested from 1823. Meaning "underwear" is recorded from 1910.
unmentionables pl (plural only)
- (plural only) undergarments, underwear, drawers
- (plural only) genitals
2016 January 24, Les Chappell, “TV: Review: The Simpsons (Classic), “Natural Born Kissers” (season nine, episode 25, originally aired 05/17/1998)”, in The Onion AV Club:
- The show understands that implied nudity is worlds funnier than actual nudity, and goes to delightful lengths to keep Marge and Homer’s unmentionables out of view.
- (plural only, obsolete) breeches, trousers
Because of a euphemism's intentional ambiguity, the perceived meaning can drift. While at first, "unmentionables" referred with Victorian hyper-sensitivity to trousers, the functional meaning drifted expediently to underwear. Currently, examples can be found where it is clear the writer or speaker means to refer expressly to the genitals.