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Adjective: rascally  rãs-ku-lee
  1. Lacking principles or scruples
    "the rascally rabble";
    - roguish, scoundrelly, blackguardly
     
  2. Playful in an appealingly bold way
    "a rascally grin";
    - devilish, roguish

See also: dishonest, dishonorable [US], dishonourable [Brit, Cdn], ludic [formal], playful