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let (one's) hair down
To drop one's reserve or inhibitions.
http://www.thefreedictionary.com/let (one's) hair down

Meaning

Behave in a free or uninhibited manner.

Origin

Letting one's hair down was a commonplace part of women's daily activities in the 17th century. The hair was normally pinned up and was let down for brushing or washing. The term used for this at the time was dishevelling. Anyone who is unkempt and generally untidy might now be described as dishevelled but then it applied specifically to hair which was unpinned. The first reference I can find which refers specifically to this is John Cotgrave's, The English treasury of wit and language, 1655:
http://www.phrases.org.uk/meanings/228000.html

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Note added at 17 mins (2010-05-10 15:42:35 GMT)
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http://www.usingenglish.com/reference/idioms/let your hair d...