Enforced by the ecclesiastical courts who had responsibility for the moral behaviour of the laity, buttock mail was levied as a fine for sexual intercourse out of wedlock.
The term mail is an old Scots word meaning a monetary tribute or payment (a loanword from Scottish Gaelic màl), from the same root as the word blackmail.
Thus, the term buttock mail literally means a monetary payment related to prostitution, referring to its being a fine for fornication, or sex outside of marriage.
The remaining jurisdiction of the ecclesiastical courts was removed in the 19th century.
[4] The normal punishment was a public confession through use of the Stool of Repentance, but payment of buttock mail commuted this sentence, allowing the convicted fornicator to avoid public humiliation.