mend one's ways
Improve one's behavior, as in Threatened with suspension, Jerry promised to mend his ways. This expression, transferring a repair of clothes to one of character, was first recorded in 1868, but 150 or so years earlier it had appeared as mend one's manners.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.