To keep it under your hat is to keep something secret. Keep it under your hat for now.
It is friendly rather than serious, so it suits news that is early rather than confidential. The picture is of something hidden on your person where nobody will look.
1phrasekeep it under your hatTo keep something secret.
“I heard on Friday, so keep it under your hat until it is announced.”
keep it under your hat is a phrase.
Practised in Idioms