let one's hair down, to

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]

To give free expression to one’s private views; to behave informally. The term alludes to the long-standing practice of women wearing their long hair pinned up in a variety of styles and taking it down only in the privacy of the bedroom. At first (mid-nineteenth century) the term was to let down the back hair; later it was simply hair. P. G. Wodehouse used it in Heavy Weather (1933): “We can take our hair down and tell each other our right names.”