unmentionables

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.

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unmentionables pl (plural only)

1. plural of unmentionable
2. (plural only) undergarments, underwear, drawers
3. (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^[1]:

     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.

4. (plural only, obsolete) breeches, trousers
   • 1922 February, James Joyce, “[Episode 13: Nausicaa]”, in Ulysses, Paris: Shakespeare and Company, […], →OCLC, part II [Odyssey], page 332:

     His little man-o'-war top and unmentionables were full of sand but Cissy was a past mistress in the art of smoothing over life's tiny troubles and and^[sic] very quickly not one speck of sand was to be seen on his smart little suit.

     Referring to a child’s sailor suit.

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.