Each week of 2017 we present an English idiom.
"keep it under your hat": don't tell anyone, keep the information a secret
Example 1: I found out from a friend at corporate that we're going to take over another company. It'll be announced next week, so keep it under your hat till then.
Example 2: We're having a small party for Simon's 50th birthday next week. It's at 4 p.m. on Friday if you want to stop by. But keep it under your hat – it's a surprise.