Etymology Attested from 1900. The phrase, meaning "intense interrogation by police," likely refers to the Third Degree of Master Mason in Freemasonry, the ceremony for which included an interrogation. Alternatively, it reflects the practice of interrogation under torture, where three degrees of torture were recognized, of increasing intensity. In other contexts, three degrees of interrogation were recognised, with torture being the third degree.
1
Informal Intensive rough interrogation in order to extract information or a confession
When I came home at 4 a.m., my dad really gave me the third degree.
In the book I use the terms "third degree" and "torture" interchangeably because "third degree" has become synonymous with torture and the interviewees often used them interchangeably.2020, Jinee Lokaneeta, The Truth Machines, University of Michigan Press, →ISBN, page 168:
Related terms
Spanish Inquisition
third-degree
adj
› At the third level of some system
1
Law The least severe level of felony, typically involving a lack of planning and/or the presence of various mitigating factors.
The jury found him guilty of third-degree murder since he had only meant to beat the clown, not to kill him.
2
Of burns An extremely severe level of damage, typically involving major scarring and damage to subcutaneous tissue as well as the epidermis.