'bestiality' originates from Latin, specifically the Medieval Latin word 'bestialitas', ultimately from 'bestia' meaning 'beast'.

'bestia' in Latin gave rise to Medieval Latin 'bestialitas', which entered Middle English (as forms like 'bestialite'/'bestialite') and eventually became modern English 'bestiality'.

Initially it referred generally to 'beastliness' or qualities of a 'beast'; over time a specialized legal and medical sense developed referring to sexual intercourse between a human and an animal, while the older sense of extreme brutality remains in figurative use.