indescribable (adj.) Look up indescribable at
1726, from in- (1) "not, opposite of" + describable (see describe). Related: Indescribably; indescribability (1797). In same sense, Old English had unasecgendlic. Indescribables for "trousers" (1819) was colloquial in England for a generation or so.
We cannot omit here to state, that, some years since, we recollect a rumour in the gallery [of the House of Commons], that Madame de Staël was sitting, en habit d'homme, in a surtout and military indescribables, listening to the debate, under the protection of Sir J. Macintosh. ["Privileges of Women," in "Retrospective Review," London, 1824]
Inexpressibles (1790) seems to have been the original and the others variations on it: Unmentionables (1806); indispensibles (1820); ineffables (1823); unutterables (1826); inexplicables (1829).