1. Expression：let one’s hair down

Explanation：To behave casually or informally. （举止随意，放松无拘束）

Example：Come on, Jim, this is a party! Let your hair down and go a little wild!

Jim，来吧，这是个派对！放松放松，狂野一点！

It's wonderful to let your hair down at the weekend after you've been working hard all week.

辛苦工作一周后在周末不拘礼节轻松一下真是太好了。

The president has to make so many official appearances that he seldom gets a chance to let his hair down and enjoy life like ordinary people.

总统要出席许多公开场合，因此他很少有机会像普通老百姓那样轻松地享受生活。

You've been working very hard lately and you look tired. Why don't you get out more and let your hair down occasionally?

你最近工作一直很辛苦，因而看上去很疲倦。你为什么不多多外出，时而放松享受一下呢？

情景对话：

A: I need to let my hair down for a few days. I always feel depressed these days.

我需要放松一下了，因为这些天我总是觉得心情沮丧。

B: Of course, you do. You have been working for 15 days without a rest. But have you decided where to go for a travel?

当然了，你已经连续工作半个月没有休息了。不过，你觉得去哪儿旅行呢？

A: I have got no idea yet.

现在，我还不知道。

2. Expression：let one’s hair down

Explanation：To talk frankly. （对某事毫无保留地说出来，把心里话说出来）

Example：Sally, we've been good friends for a long time. But lately I get the idea you are very much upset with me. I wish you'd let your hair down and tell me what's wrong to make you feel this way.

萨莉，我们多年来一直是好朋友，可是最近我感到你对我非常不高兴。我希望你能够坦率地告诉我，到底是些什么事让你这么不高兴。

It is wonderful that we can let our hair down at the meeting this afternoon.

今天下午的会议上，我们能够坦诚交谈太棒了。