|ənbə̇|nōn, -bē|- adjective
or un·be·knownst -ōnzt, -ōn(t)st
Etymology: unbeknown from un- (I) + obsolete English beknown known, familiar, from Middle English beknowen, past participle of beknowen to become acquainted with, from Old English becnāwan, from be- + cnāwan to know; unbeknownst irregular from unbeknown — more at know
: happening without one's knowledge : unknown — usually used with to
two elderly women unbeknown to anybody — J.H.Holmes