keep something under your hat
keep (something) under (one's) hat
To not reveal or discuss something publicly. Keep this under your hat for now, but Greg is definitely the one getting the promotion.
keep something under your hatINFORMAL
If you keep something under your hat, you do not tell anyone else about it. Very few people know, so keep it under your hat. Look, if I tell you something will you promise to keep it under your hat? Note: This was a slogan used to promote security in Britain during the Second World War.