Third degree
In medical nomenclature, “third-degree burns” are the most intense on the spectrum of first-, second-, and third-degree. In criminal justice terminology, a third-degree offense is the least serious type of infraction, as in a felony “in the third degree.” Then there’s the idiomatic noun sense of third degree meaning “intensive questioning or rough treatment.” This sense arose in the late 1800s and may be related to a degree, or position, in the fraternal organization of Freemasonry called master mason, which is the highest of the three ranks in the order. Attaining this title requires passing a difficult interrogation test which may have influenced the idiom.