Etymology
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unbeknownst (adj.)

1833, vulgar formation from unbeknown (1630s). No clear reason for the -st, but since 19c. this has become the dominant form.

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Definitions of unbeknownst
1
unbeknownst (adv.)
without someone's knowledge;
unbeknownst to me, she made all the arrangements
Synonyms: unbeknown
2
unbeknownst (adj.)
(usually used with `to') occurring or existing without the knowledge of;
she had been ill for months, unbeknownst to the family
Synonyms: unbeknown
From wordnet.princeton.edu