means Not known, identified, or established — referring to something whose nature, identity, or value has not been determined.

from A plain Old English build: the prefix 'un-' meaning 'not,' bolted onto 'known,' the past participle of 'cnawan,' to know. 'Cnawan' is deep Germanic stock with cousins across the language family and a distant relative in Latin 'gnoscere' and Greek 'gignoskein' — the same ancient root that gives us 'gnosis' and 'recognize.' So 'unknown' is, quite literally, the negation of knowing, built brick by honest brick.