/ ˌʌnbɪˈnəʊn; NAmE -ˈnoʊn/ (also less frequent un·be·knownst / ˌʌnbɪˈnəʊnst; NAmE -ˈnoʊnst/) adjective
unbeknown to sb ( formal ) without the person mentioned knowing :
Unbeknown to her they had organized a surprise party.
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WORD ORIGIN
mid 17th cent.: from un- not + archaic beknown known .