If someone plans your birthday party unbeknownst to you — that is, you're completely unaware of it — it will probably be a surprise party.

Used as an adjective or adverb, unbeknownst is descended from unbeknown (1848), which combines the prefix un- ("not") with be ("by, about") and know. Sometimes the FBI might be secretly working on a case, unbeknownst to the CIA, which is also secretly working on it. Imagine their frustration when everyone finds out they could have shared information and work, while saving time and manpower.

Definitions of unbeknownst

adj (usually used with `to') occurring or existing without the knowledge of

“she had been ill for months, unbeknownst to the family”
not known

adv without someone's knowledge

unbeknownst to me, she made all the arrangements”

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