Definition of unbeknownst adjective from the Oxford Advanced American Dictionary

 

unbeknownst

 adjective
adjective
NAmE//ˌʌnbɪˈnoʊnst//
 
(also unbeknown
NAmE//ˌʌnbɪˈnoʊn//
 
)
unbeknownst to somebody(formal)
 
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without the person mentioned knowing Unbeknown to her they had organized a surprise party.