Literary or formal word describing extreme cruelty, violence, or ferocity. Not common in everyday speech; more likely in written descriptions of brutal acts or fierce animals.
その猛悪な犯罪に町中が震え上がった。
The whole town trembled at the atrocious crime.
There is a record that a ferocious beast attacked the village.
Compound of 猛 (もう, 'fierce, violent') and 悪 (あく, 'evil, bad'). The exact historical derivation is uncertain, but the combination straightforwardly conveys extreme ferocity and wickedness.