Let your hair down

Whats the origin of idiomatic expression of 'let your hair down'?

My guess -- grown women used to wear their hair up in a bun or whatever. Hair was only "let down" at night or when they were going to wash it.

Close. The point of the idiom is that it was improper for a grown woman to appear in public with her hair loose, so she could only let her hair down in an informal situation when only family or intimate friends were present. A young man might ask his fiancee if he could see her with her hair down - that was quite a sexy request, which he couldn't decently make before they were engaged. (VSD)