One theory traces the phrase back to Freemasonry, where members rise through three levels, culminating in the Master Mason or third degree. This final step involved a complex ritual that may have included intense questioning, leading some to associate it with serious grilling.

Another theory points to early 20th-century police work, where "the third degree" referred to the harshest level of interrogation, using no-nonsense tactics meant to force a confession.

Detective Thomas F. Byrnes of New York helped popularize the phrase in the late 1800s with his aggressive style. Thanks to the press, “giving someone the third degree” quickly became shorthand for any intense questioning.