let your hair down
Meaning: If you let your hair down, you enjoy yourself by doing whatever you feel like doing and not worrying about what other people might think.
- It's hard to imagine Francine letting her hair down and having fun, but Helen said she's like another person when she's not at work. She's funny and lively and she knows how to have a good time.
- Come on, George! It's Saturday night so why don't you stop being so serious and sensible and let your hair down for a change?