Origin: In the seventeen and eighteenth-century Paris, members of the nobility were obliged to wear elaborate hairdos - stacked preposterously high - which took several hours to prepare. At the end of a long day of intriguing and ingratiating at the Palace of Versailles, they would go home to relax and the first thing that they would do was to let down their hair.
Meaning: Behave in an uninhibited or relaxed manner or let your hair down and just have some fun.
Synonyms: enjoy oneself, have a good time, have fun, make merry, let oneself go,
Informal: have a ball, whoop it up, paint the town red, live it up, have a whale of a time, let it all hang out.
Quotes: "Even the chairman of the board has to let his hair down once in a while."