Diabolical means "evil." This is a strong word. Too much math homework might seem unfair, but it probably isn't diabolical.

Diabolical is related to the Spanish word diablo, which means "devil." In many religions and mythologies, you can't get any more evil than a devil, so diabolical is a powerful term, stronger than even despicable, which is another strong word for things that are hateful. Diabolical things are also intentional. If you got hit by lightning, that's awful and harmful, but it's not diabolical. If someone deliberately runs you over with a car, that's much more diabolical.

Definitions of diabolical
  1. adjective
    showing the cunning or ingenuity or wickedness typical of a devil
    “the diabolical expression on his face”
    synonyms: devilish, diabolic, mephistophelean, mephistophelian
    morally bad or wrong
  2. adjective
    extremely evil or cruel; expressive of cruelty or befitting hell
    synonyms: demonic, diabolic, fiendish, hellish, infernal, satanic, unholy
    morally bad or wrong
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