an intense interrogation, originally referring to police/government attempts to get someone to confess by torture or beating. (Now often used in a sarcastic sense: "What is this, the third degree?" when someone asks a nosy question.)

Exactly where the phrase comes from is disputed. Some say it is from Masonic rituals where a disorienting questioning was part of the ceremony to be initiated into the Third Degree level; other sources say it has nothing to do with Masons and comes from "three degrees of torture" instituted by 18th century Austrian Empress Maria Theresa as the limit that any accused person could be made to undergo, or that it comes from American police department slang in the late 19th century for three levels of interrogation.