The Oxford Thesaurus An A-Z Dictionary of Synonyms - mysteriousadj.

1) puzzling, enigmatic, baffling, insoluble, unsolvable, bewildering, confounding, confusing, perplexing, mystifying, weird, bizarre, strange, uncanny, curious: A mysterious force makes the saucer rise off the ground and fly at incredible speeds. 2) cryptic, arcane, secret, inscrutable, covert, hidden, furtive, unclear, dark, concealed, occult, inexplicable, incomprehensible, mystic(al) , unknown, unfathomable, mystical, recondite, abstruse: It is mysterious how the agency accomplishes its ends.