“let one's hair down”直译为“把头发放下来”，但它所表达的真实意思是“ 放松下来，不再拘束，放松自己”。

【例句】
We've been working so hard on this project, let's go out this evening and let our hair down at a karaoke bar！
我们为了这个项目辛辛苦苦地干了那么久，今天晚上让我们去唱卡拉ok放松一下吧！

The president has to make so many official appearances that he seldom gets a chance to let his hair downand 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。