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Adjective: insidious  in'si-dee-us
  1. Beguiling but harmful
    "insidious pleasures"
  2. Intended to entrap
  3. Working or spreading in a hidden and usually injurious way
    "glaucoma is an insidious disease";
    - pernicious, subtle

See also: dangerous, harmful, seductive, unsafe

Encyclopedia: Insidious