Idiom: Let your hair down
OED definition: Behave uninhibitedly
Example: This weekend I’m going to let my hair down!
This term dates back to the 17th century, when women would typically wear their hair pinned up in an elaborate style on the top of their heads. Only once in the privacy of her bedchamber, would a woman take her hair out of the style that she had worn during the day and allow the hair to be loose and down around her shoulders. She would literally ‘let her hair down’. This expression has therefore since come to reflect the relaxed and free feelings a woman had at the end of the day when she had taken her hair out of the stiff and often uncomfortable style that she had been wearing.
Interestingly, the term used at the time for this process was ‘dishevelling’, with one of the first print references in John Cotgrave’s ‘The English treasury of wit and language’ in 1655. The current meaning of ‘dishevel’ has taken on a slightly more negative connotation ‘to make a person’s hair or clothes untidy’ (OED), which has deviated slightly from the original meaning.