unseen (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.

Middle English sein, "visible, able to be seen with the eyes; plain, clear, manifest," from Old English gesegen, gesewen, past participle of seon (see see (v.)). From c. 1200 as "perceived, discovered." From c. 1300 as "experienced, undergone." To have seen everything as a hyperbolic expression of astonishment is from 1941 (the phrase itself is older, in "Gatsby," etc.).

He that has seen one thing hath seen all things ; for he has got the general idea of something. [Locke, 1706]

The saw or vulgar maxim about children being best seen and not heard (by 1816) was previously of maids specifically (mid-15c.).

Well, at length my wish was in part gratified—lady Cowley was announced. It has been said that women, like children, should be "seen and not heard." I am no advocate for dumb dolls, yet I object to catching the voice through long passages ere one sees the party, and in the present instance ... (etc.) ["The Spinster's Journal," vol. 1, by 'A Modern Antique,' London: 1816]