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)
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.