/ ˌdaɪəˈbɒlɪkl; NAmE -ˈbɑːl-/ adjective
1.
( informal , especially BrE ) extremely bad or annoying
SYN terrible :
The traffic was diabolical.
2.
(also less frequent dia·bol·ic / ˌdaɪəˈbɒlɪk; NAmE -ˈbɑːl-/) morally bad and evil; like a devil
► dia·bol·ic·al·ly / -kli; NAmE / adverb