'beastly' originates from Middle English, formed from the noun 'beast' plus the adjectival suffix '-ly'. 'beast' ultimately originates from Latin, specifically the word 'bestia', where 'bestia' meant 'animal'.

'beast' passed into English via Old French 'beste' from Latin 'bestia'; the adjective 'beastly' was formed in Middle English by adding '-ly' to 'beast' and eventually became the modern English word 'beastly'.

Initially it meant 'of or like a beast' (animal-like); over time it developed senses of 'savage' or 'brutal', later acquiring colloquial senses 'very unpleasant' and as an intensifier 'very'.