Use the adjective diabolic to describe someone who acts in a terribly cruel way. A diabolic boy might pick up a cat by its tail and swing it around.

When people are diabolic, they're evil. The horrible dictator who has thousands of civilians killed commits a diabolic act, and people who abuse children are also diabolic. The bad guy in a fairy tale is always diabolic. You can use diabolic interchangeably with the slightly more common diabolical. Both words have a Greek root, diabolikos, which means "devilish" and comes from diabolus, or "devil."

Definitions of diabolic
  1. adjective
    showing the cunning or ingenuity or wickedness typical of a devil
    “the cold calculation and diabolic art of some statesmen”
    synonyms: devilish, diabolical, mephistophelean, mephistophelian
    morally bad or wrong
  2. adjective
    extremely evil or cruel; expressive of cruelty or befitting hell
    “fires lit up a diabolic scene”
    synonyms: demonic, diabolical, fiendish, hellish, infernal, satanic, unholy
    morally bad or wrong
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