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Adjective: unbeknownst  ,ún-bi'nownst
  1. (usually used with ‘to’) occurring or existing without the knowledge of
    "she had been ill for months, unbeknownst to the family";
    - unbeknown
Adverb: unbeknownst  ,ún-bi'nownst
  1. Without someone's knowledge
    "unbeknownst to me, she made all the arrangements";
    - unbeknown

See also: unknown