Source : Webster's Revised Unabridged Dictionary (1913)

Mysteriously \Mys*te"ri*ous*ly\, adv.
   In a mysterious manner.

Source : WordNet®

mysteriously
     adv 1: in a mysterious way; "the book had been mysteriously moved
            from its customary place"
     2: in a cryptic manner; "we will meet again," he said
        cryptically [syn: {cryptically}, {enigmatically}]