let one's hair down

Meaning: 羽を伸ばす、本音を出す、くつろぐ

For Example:

‣ After a long day at work, everyone let their hair down at the company's Christmas party.

仕事で長い一日を過ごした後、みんな会社のクリスマスパーティーで羽を伸ばした。

‣ She let her hair down and told me about herself when she realized we were friends.

うちらが友達やったと気づいた時、彼女は本音で自身のことを話してくれた。

Synonym:

to be relaxed

behave informally