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.”