unmentionable adj.

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

"that can be or is worthy to be mentioned," 1630s, from mention (v.) + -able.

1620s, from in- (1) "not" + expressible (see express (v.)). Inexpressibles "trousers" is from 1790. Related: Inexpressibly.

I have retain'd the word BREECHES, as they are known by no other name amongst country folk.--The change from vulgarity to refinement, in cities and towns, has introduced other appellations; there they are generally called SMALL CLOTHES, but some ladies of high rank and extreme delicacy call them INEXPRESSIBLES. [footnote in "Poems Miscellaneous and Humorous," by Edward Nairne, Canterbury, 1791]

Inexpressibles is the earliest recorded and thus seems to have begotten the trend: Unmentionables (1806); indispensibles (1820); ineffables (1823); unutterables (1826); innominables (1827); and inexplicables (1829) followed.