keep sth. under one’s hat千万不要简单地理解为“把...藏在帽子下面”，所以这个短语的实际意思是“保密；别告诉别人”。

【例句】
She is in a bad mood recently because she and her husband divorced several days ago. Please keep it under your hat.
她最近心情不好，因为她和她老公前几天离婚了。这事你一定要保密啊！

If I tell you something, will you promise to keep it under your hat?
如果我告诉你一些事情，你能保证不告诉别人吗？