unbeknown

Estimated CEFR level: C2 — Proficiency

Estimated from word frequency; not an official CEFR classification.

Definition

adjective (usually used with `to') occurring or existing without the knowledge of
adverb without someone's knowledge

From un- + beknown.

It would be unbeknown to the Queen's Highness! — The Fifth Queen Crowned, Ford Madox Ford
He came again, and they met often, unbeknown to the decorous Madame de Mezières. — Mr. Standfast, John Buchan
Now, it so chanced that, unbeknown to the cripple, a prim-looking stranger had overheard part of his story. — The Confidence-Man: His Masquerade, Herman Melville

unbeknownst

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