Dictionary.net

A carnal copulation against nature; and this is either by the confusion of species,

Crim. law. The detestable crime of having commerce contrary to the order of nature, by mankind with mankind, or with brute beasts, or by womankind with brute beasts. 3 Inst. 58; 12 Co. 36; Dane's Ab. Index, h. t.; Merl. Repert. mot Bestialie. This is a highly penal offence.

Said to have been introduced by the Bulgarians: hence the name. A carnal copulation against nature, as of a man or woman with any animal; or of a man with a man, or a man unnaturally with a woman. The unnatural crime.