a piece of (one's) mind
One's true thoughts or feelings about something, especially angry or frustrated criticism. Boy, the boss really gave me a piece of his mind during my performance review. This has to have been the worst service I've ever had a restaurant. I'm going to let the waiter have a piece of my mind!
piece of one's mind
Frank and severe criticism, censure, as in Chuck was furious and gave him a piece of his mind. The word piece here is used in the sense of "portion," but the portion of the mind alluded to always has a negative opinion. [Second half of 1500s]
a piece of (one's) mind
Frank and severe criticism; censure.