Word Origin & History
"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony. Third degree as a measure of severity of burns (most severe) is attested from 1866, from Fr. (1832); in Amer.Eng., as a definition
of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.