This phrase has several meanings. For mystery readers, it means long and intense questioning by police to obtain information or a confession. For the medical profession, it refers to burns of the most severe kind, affecting tissue below the skin. For lawyers, it means the least serious category of a crime, especially murder.
The ‘third degree’ is sometimes a euphemism for torture (physical or mental) used to extract confessions. In 1931, the Wickersham Commission found that use of the third degree was widespread in the United States. The use of the third degree was technically made illegal after the Wickersham report. However, the interrogation method used now is seen by many as merely a psychological version of the third degree. Psychological torture is just as capable as physical torture of extracting a false confession.
There are also several possible origins for the phrase:
— The term may have been coined by 19th century New York City Police detective Thomas F. Byrnes, perhaps as a pun on his name, as in third degree burns.
— The Knights of Columbus have a third-degree ceremony which is required to advance to that level, seen as rigorous.
— In Masonic lodges (which originated hundreds of years ago in local fraternities of stonemasons) three degrees of membership exist. To achieve the third degree, that of master mason, the candidate is subjected to an interrogation and other rites more physically challenging than the first two.
— Progressive degrees of torture used to extract confessions during the Spanish Inquisition.
— Natural philosophy assigns the ‘third degree’ to the penultimate level of intensity. Thus, in 1578, Henry Lyte’s translation of Dodoens’ Niewe herball or historie of plantes includes a description of rue: “Rue is hoate and dry in the thirde degree.” A few years later, in 1602, Shakespeare used it to describe drunkenness in Twelfth Night: “For he’s in the third degree of drinke: hee’s drown’d: go looke after him.”
— The great difficulty of solving third-degree polynomial equations by comparison with those of second degree.
Solving third-degree polynomial equations certainly sounds like torture to me!