Lat. The sixth (book) of the decretals; the sext, or sixth decretal. So called because append-ed, in the body of the canon law, to the five books of the decretals of Gregory IX.; it consists of a collection of supplementary de-cretals, and was published A. D. 1298. Butl. Hor. Jur. 172; 1 BL Comm. 82
Source: Black's Law Dictionary 2nd Ed (1910)
