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let one's hair down

let one's hair down idiom

Also, let down one's hair. Drop one's reserve or inhibitions, behave casually or informally, as in Whenever the two sisters get together, they let their hair down and discuss all their problems. This expression alludes to the practice of women taking down their pinned-up long hair only in the privacy of the bedroom. [c. 1900]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.