3.to keep something under your hat
不论是把一封信放在帽子底下，还是戴上一顶帽子把自己的秃顶遮起来，帽子都算的上是一个藏东西的好地方。而to keep something under your hat的意思就是：我要告诉你一个秘密，但是千万不要告诉任何人。
例句：mary told me her husband is seeing another woman and she's thinking about leaving him but please keep it under your hat.
玛丽告诉我她的丈夫有外遇。玛丽正在考虑跟她丈夫分手。不过，请你千万不要对别人说。
又如：can you keep a secret? the boss's secretary just told me he's leaving for another job. she told me not to tell anybody else, so keep it under your hat.
你能保密吗？我们老板的秘书刚刚告诉我老板要上别处去工作了。他的秘书叫我不要对任何人说，所以你得保密呀！