Diabolical

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late Middle English (as diabolic ): from Old French diabolique or ecclesiastical Latin diabolicus, from diabolus ‘devil’; the form diabolical dates from the early 16th century.

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From diabolic +‎ -al.

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diabolical (adj.)

c. 1500, "pertaining to the devil," from diabolic + -al (1). Meaning "befitting the devil" is from 1540s. Related: Diabolically.