Diabolical
WordNet
adjective
(1) Extremely evil or cruel; expressive of cruelty or befitting hell
"Something demonic in him--something that could be cruel"
"Fires lit up a diabolic scene"
"Diabolical sorcerers under the influence of devils"
"A fiendish despot"
"Hellish torture"
"Infernal instruments of war"
"Satanic cruelty"
"Unholy grimaces"
(2) Showing the cunning or ingenuity or wickedness typical of a devil
"Devilish schemes"
"The cold calculation and diabolic art of some statesmen"
"The diabolical expression on his face"
"A mephistophelian glint in his eye"
WiktionaryText
Etymology
First attested between 1350 and 1400 from Middle English diabolik, from Middle French diabolique, from Late Latin diabolicus, from Greek , from diabolos. See devil.