Word Study
unbeknown
OXFORD DICTIONARY
unbeknown, adj. (also unbeknownst) (foll. by to) without the knowledge of (was there all the time unbeknown to us).
Etymology
UN-(1) + beknown (archaic) = KNOWN
THESAURUS
unbeknown
able, enigmatic, incalculable, incognizable, mysterious, puzzling, sealed, strange, unapparent, unapprehended, unascertained, uncharted, unclassified, undisclosed, undiscoverable, undiscovered, undivulged, unexplained, unexplored, unexposed, unfamiliar, unfathomed, unheard, unheard-of, unidentified, uninvestigated, unknowable, unknown, unperceived, unplumbed, unrevealed, unsuspected, untouched, virginFor further exploring for "unbeknown" in Webster Dictionary Online