un·be·known (ŭn'bĭ-nōn') adj. Occurring or existing without the knowledge of; unknown: a crisis unbeknown to us.
[un-1 + obsolete beknown, known (from Middle English beknowen, past participle of beknowen, to get to know, from Old English becnāwan : be-, be- + cnāwan, to know; see know).]