WordWeb Online

1. Of an obscure nature
   "the new insurance policy is written without cryptic or mysterious terms"; "in its mysterious past it encompasses all the dim origins of life"
   - cryptic, cryptical, deep, inscrutable, mystifying
    
2. Having an import not apparent to the senses nor obvious to the intelligence; beyond ordinary understanding
   "mysterious symbols"
   - mystic, mystical, occult, secret, orphic

See also: esoteric, incomprehensible, inexplicable, mysteriously, mystery

Encyclopedia: Mysterious