Where did the phrase "third degree" (referring to intense interrogation) originate?
Additionally, how did "grill" come to have its related meaning?
Wikipedia has an article on third degree interrogation. Here are its hypotheses as to its origin:
As an aside, you may be interested in this popular and related question.
Phrases.org seems more definite about the origin:
Grill meaning to subject to severe and persistent cross-examination is a metaphor. Imagine the examinee on a grill, being intensely "cooked" through by the questioning. Etymonline says its first usage in this sense is attested in 1894.
According to the OED, it's ‘an interrogation of a prisoner by the police involving the infliction of mental or physical suffering in order to bring about a confession or to secure information. originally U.S.’ First recorded in 1880. ‘Third degree’more generally has meanings related to the greatest degree of intensity, so it’s not hard to see how the meaning was transferred to an interrogation.
The first recorded use of ‘grill’ meaning ‘to subject to severe questioning’ is dated 1894. Its use in that sense seems to stem directly from the idea of placing something over a fire, and hence tormenting with heat.