“The cake is for Susan’s birthday, but keep it under your hat. We’re throwing a surprise party.” 「このケーキはスーザンの誕生日のために用意したんだけど秘密にしててね。サプライズパーティーをやるんだ。」


To keep something under your hat is to keep it a secret. If someone tells you a secret, you can’t let anybody else know! Keep the information under your hat where no one can see it. This phrase is very old, but some people think it comes from medieval archers who kept bowstrings under their hats.