Two expressions, peace of mind and a piece of one’s mind – they sound almost the same, but they are in fact very different!
PEACE OF MIND:
Peace of mind describes being calm, without worries, having a feeling of tranquility and security. We often talk about having peace of mind or things that give us peace of mind: For example-
I’d rather make less money and have peace of mind than have a high-earning, high-stress job.
Our home security system gives us peace of mind because we know our house is protected.
GIVE SOMEONE A PIECE OF YOUR MIND:
The expression gives someone a piece of your mind means to express your honest feelings, when those feelings are angry, criticizing, or negative in some way. When you give someone a piece of your mind, you don’t make your criticism more polite or diplomatic; instead, you express your negative feelings in strong and direct words.
After an employee was late to work for the third time in a week, the boss gave him a piece of his mind. (For example, he said something like “Lateness is completely unacceptable. If you’re late one more time, you will be fired.”)