Beastly means behaving like a wild animal: cruel, savage, or brutish. When someone acts in a beastly way, they're being mean and uncivilized, treating others roughly without kindness or consideration. A bully who torments younger students is acting beastly. A dictator who rules with cruelty shows beastly behavior.

The word comes from beast, which means an animal, and suggests someone has abandoned the thoughtfulness and self-control that make us human. Think of the difference between a trained dog that obeys commands and a wild wolf that attacks without warning: that's the contrast between civilized behavior and beastly conduct.

In British English, people sometimes use beastly more lightly to mean simply unpleasant or disagreeable, like beastly weather for a cold, rainy day or a beastly headache for a particularly bad one. But the core meaning remains: something harsh, unpleasant, and uncomfortable.