Diabolical means worthy of the Devil himself. A diabolical plan, diabolical cunning, diabolical cruelty.
In British speech it has a second, much lighter life meaning simply appalling, so diabolical weather or a diabolical performance means dreadful with no devils involved. The tone tells you which one is meant.
1adjectivediabolicalWorthy of the Devil himself: fiendishly wicked, or cruelly cunning.
“The villain came up with a diabolical plan to poison the city's water.”
diabolical is a quality.
From diabolic + -al.
Etymology from Wiktionary, CC BY-SA