mysterious

mysterious adjective abstruse, arcane, arcanus, baffling, cabalistic, clandestine, cloaked, coded, covert, cryptic, dark, disguised, enigmatic, esoteric, furtive, hidden, impenetrable, ineffable, inexplicable, inscrutable, magical, masked, mystic, mystical, mystifying, obscure, occult, occultus, oracular, perplexing, preternatural, privy, puzzling, recondite, runic, screened, secret, secretive, secretus, sphinxian, sphinx-like, stealthy, strange, supernatural, supernormal, surreptitious, transcendental, umbrageous, unaccountable, uncanny, undeciphered, undercover, undisclosed, unexplainable, unexplained, unintelligible, unknown, unrevealed, untold, veiled, weird

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See also: abstruse, ambiguous, confidential, covert, elusive, enigmatic, esoteric, evasive, furtive, hidden, inapprehensible, incomprehensible, indefinable, indefinite, indistinct, ineffable, inexplicable, inscrutable, obscure, oracular, peculiar, private, privy, problematic, recondite, secret, surreptitious, uncanny, uncertain, undisclosed, vague