Source : Webster's Revised Unabridged Dictionary (1913)

Unbeknown \Un`be*known"\, a.
   Not known; unknown. [Colloq.]

Source : WordNet®

unbeknown
     adj : (usually used with `to') occurring or existing without the
           knowledge of; "a crisis unbeknown to me"; "she had been
           ill for months, unbeknownst to the family" [syn: {unbeknown(p)},
            {unbeknownst(p)}]
     adv : without someone's knowledge; "unbeknownst to me, she made
           all the arrangements" [syn: {unbeknownst}]