The definition of not mince words is:
(idiom) directly criticize someone without being polite
Student: How did I do on my exam, teacher?
Teacher: I will not mince words, your speaking was terrible. Your grammar is below-average, and you don’t know any vocabulary.
Rebecca’s boss did not mince words when he told Rebecca that she was lazy, always late, and rude to other co-workers.
Boyfriend: I’m breaking up with you.
Boyfriend: It’s not important.
Girlfriend: Tell me! Don’t mince words.
Boyfriend: I’m gay.
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