The meaning of "give someone a piece of your mind" is:
The definition of "give someone a piece of your mind" is:
(idiom) to scold or criticize someone strongly
The teacher is going to give the student a piece of her mind, because he is always late.
Anthony’s mother gave him a piece of her mind because he was rude during Christmas dinner.
William’s girlfriend is giving William a piece of her mind, as he forgot their anniversary.