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Adjective: unbeknown  ,ún-bi'nown
  1. (usually used with 'to') occurring or existing without the knowledge of
    "a crisis unbeknown to me";
    - unbeknownst
Adverb: unbeknown  ,ún-bi'nown
  1. Without someone's knowledge
    "unbeknown to me, she made all the arrangements";
    - unbeknownst

See also: unknown