keep sth. under one's hat 字面意思是：把...藏在帽子下面，但它真正表达的意思是：保密；别告诉别人。

【例句】
Mary and her husband have divorced several days ago.Please keep it under your hat.
玛丽和她老公前几天离婚了。这事你一定要保密啊！

You must keep it under your hat for me.
这事你一定要帮我保密。