The adj mysterious has 2 senses (first 2 from tagged texts)
1. (14) cryptic, cryptical, deep, inscrutable, mysterious, mystifying -- (of an obscure nature; "the new insurance policy is written without cryptic or mysterious terms"; "a deep dark secret"; "the inscrutable workings of Providence"; "in its mysterious past it encompasses all the dim origins of life"- Rachel Carson; "rituals totally mystifying to visitors from other lands")
2. (4) mysterious, mystic, mystical, occult, secret, orphic -- (having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding; "mysterious symbols"; "the mystical style of Blake"; "occult lore"; "the secret learning of the ancients")