Make a clean breast of it means to confess fully, honestly, and without holding anything back. It is usually used when someone finally tells the whole truth after hiding something.
The wording sounds unusual today because breast is an older way of referring to the chest or heart as the place where private feelings and secrets are kept. In this idiom, clean means clear or emptied out. So the image is of clearing the heart of what has been hidden.
This is why the expression came to mean complete confession, not just a small admission. It often suggests relief, guilt, or the end of secrecy.
- He made a clean breast of it and admitted he had altered the numbers.
- After weeks of worry, she made a clean breast of it and told her friend the truth.
The phrase is still understood in modern English, though it sounds a little old fashioned. In everyday speech, many people would now say come clean, confess everything, or tell the whole truth. Still, knowing the older idiom helps you understand books, articles, and conversations where it appears.