c. 1300, "strange, unfamiliar" (of persons, places), from un- (1) "not" + past participle of know (v.). Compare Old English ungecnawen. In reference to facts, "not discovered or found out," it attested from early 14c. The noun meaning "unknown person" is recorded from 1590s; the unknown "that which is unknown" is from 1650s.
Entries linking to unknown
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.
Old English cnawan (class VII strong verb; past tense cneow, past participle cnawen), "perceive a thing to be identical with another," also "be able to distinguish" generally (tocnawan); "perceive or understand as a fact or truth" (opposed to believe); "know how (to do something)," from Proto-Germanic *knew- (source also of Old High German bi-chnaan, ir-chnaan "to know"), from PIE root *gno- "to know."
For pronunciation, see kn-. Once widespread in Germanic, the verb is now retained there only in English, where it has widespread application, covering meanings that require two or more verbs in other languages (such as German wissen, kennen, erkennen and in part können; French connaître "perceive, understand, recognize," savoir "have a knowledge of, know how;" Latin scire "to understand, perceive," cognoscere "get to know, recognize;" Old Church Slavonic znaja, vemi). The Anglo-Saxons also used two distinct words for this, the other being witan (see wit (v.)).
From c. 1200 as "to experience, live through." Meaning "to have sexual intercourse with," also found in other modern languages, is attested from c. 1200, from the Old Testament (Genesis iv.1). Attested from 1540s in colloquial phrases suggesting cunning or savvy (but often in the negative); to not know one's ass from one's elbow is from 1930.
As far as (one) knows "to the best of (one's) knowledge" is late 14c. Expression God knows is from c. 1400. To know too much (to be allowed to live, escape, etc.) is from 1872. To know better "to have learned from experience" is from 1704.
You know as a parenthetical filler is from 1712, but it has roots in 14c. You know as a euphemism for a thing or situation unmentionable is from 1867; you-know-who for a person it is thought best not to name (but implying the hearer knows) is from 1840.
As an expression of surprise, what do you know attested by 1914. Don't I know it in the opposite sense ("you need not tell me") is from 1874. You never know as a response to something unexpected is attested from 1924.
an unknown writer
an unknown source
an unknown amount
an unknown island
a poem by an unknown author
things obscurely felt surged up from unknown depths
don't let anyone unknown into the house
they came like angels out the unknown