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Someone who has no real place or purpose in a situation, likened to a superfluous extra wheel on a two-wheeled vehicle. When Kelly invited me to go to the movies, I didn't know that her boyfriend would be joining us. I felt like a third wheel the entire night.
the third degree
Intense or thorough interrogation, scrutiny, or questioning. Kayla's boyfriend is so controlling, always giving her the third degree if she goes anywhere or sees anyone without him. I got the third degree from the police regarding my whereabouts during the crime.
(give somebody) the ˌthird deˈgree(informal) question somebody for a long time and in a thorough way; use threats or violence to get information from somebody: The soldiers were given the third degree in order to make them reveal the information. ♢ Why are you giving me the third degree?This expression comes from Freemasonry (= a secret society). In order to reach the highest level of the organization and become a Third Degree Mason, members are interrogated.
n. an extra person; a person who gets in the way. (see also spare tire.) Well, let’s face it. We don’t need you. You are a third wheel.