late 14c., "the nature of beasts," from bestial + -ity. Meaning "indulgence in beastly instincts" is from 1650s; sense of "sexual activity with a beast" is from 1611 (KJV).
bestiality bes·ti·al·i·ty (běs'chē-āl'ĭ-tē, bēs'-)n.
The quality or condition of being an animal or like an animal.
Conduct or an action marked by depravity or brutality.
Sexual relations between a human and an animal.