What is Sextus Decretalium?

Legal Definition
Lat. The sixth (book) of the decretals; the sext, or sixth decretal. So called because appended, in the bsdy of the canon law, to the five books of the decretals of Gregory IX.; it consists of a collection of supplementary decretals, and was published A. D. 1298. Buth Hor. Jur. 172; 1 Bl. Comm. 82.
-- Black's Law Dictionary