Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
"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 French (1832); in American English, as a definition of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.
Wiktionary
n. (context idiomatic English) intensive rough interrogation in order to extract information or a confession
WordNet
n. interrogation often accompanied by torture to extort information or a confession
Wikipedia
Third Degree is a 1986 album by Johnny Winter and the final of the trilogy he made for Alligator Records. For the occasion Winter temporarily reunited with Tommy Shannon and Uncle Red Turner, who used to be his first backing band. Another notable guest on the record is Doctor John, who performed on "Love, Life and Money" and "Tin Pan Alley". Winter also included a couple of solo acoustic numbers, practicing for months with a National Steel Guitar.
The third degree is a euphemism for torture ("inflicting of pain, physical or mental, to extract confessions or statements"). In 1931, the Wickersham Commission found that use of the third degree was widespread in the United States. No one knows the origin of the term, but there are several hypotheses. The use of the third degree was technically made illegal after the Wickersham report. However, the interrogation method known as the Reid technique, which is now widely used by law enforcement in the U.S., is seen by many as simply a psychological version of the third degree in that it is equally capable of extracting a false confession through coercion when abused by police.
Usage examples of "third degree".
The driving snow, so heavy as to reduce visibility to virtually zero, had an uncommonly chilling effect on the torpedo boat, which would have been of no concern to those in an adequately central-heated boat but on this particular one the central-heating unit, as became practically everything else aboard, was functioning at about only one-third degree efficiency and, moreover, had been of a pathetically ancient design in the first place so that for the shivering passengers - and crew - the snow had become a matter for intense concern.
We find it the {29} basis of many important initiations, notably the Third Degree in Masonry, and the 5 Degree = 6Square ceremony of the G.
Unlike people, the Lizards didn't seem to go in for the third degree.
Never let an idea pass through your mind without giving it the third degree.