What is Jac?

Legal Definition
An abbreviation for "Jacobus," the Latin form of the name James; used principally in citing statutes enacted In the reigns of the English kings of that name; e. g., "St. 1 Jac. II." Used also in citing the second part of Croke's reports; thus, "Oro. Jac." denotes "Croke's reports of cases in the time of James I."
-- Black's Law Dictionary
Legal Definition
James, King of England.
-- Ballentine's Law Dictionary