unbeknown

[uhn-bi-nohn]
adjective
unknown; unperceived; without one's knowledge (usually followed by to ).
Also, unbeknownst [uhn-bi-nohnst] .


Origin:
1630–40; un-1 + beknown (late Middle English beknowe, past participle of beknowen); see be-, known

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unbeknown (ˌʌnbɪˈnəʊn)
 
adv (foll by to)
1.  Also (esp Brit): unbeknownst without the knowledge (of a person): unbeknown to him she had left the country
 
adj (usually foll by to)
2.  rare not known (to)
 
[C17: from the archaic beknown known; see be-, know]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Etymonline
Word Origin & History

unbeknownst
1848, vulgar formation from unbeknown (1636). No clear reason for the -st, but since 19c. this has become the dominant form.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
He said he wanted to get there in plenty of time, and unbeknownst to him, he
  succeeded all too well.
One certainty is that my dean adeptly choreographed the event, unbeknownst to
  me until the curtain had already fallen.
Unbeknownst to buyers of insurance, it seems, they have been buying their
  products from a cartel.
Unbeknownst to many, consumer stereo products and speakers often adjust
  frequencies in the sound to make music more exciting.
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