To keep something under your hat is to keep a secret to yourself. Keep something under your hat for now.
It suits news that is early rather than confidential, which is a real distinction. The picture is of hiding something on your person where nobody would look.
1phrasekeep something under your hatTo keep a secret to yourself.
“Keep something under your hat until it is announced on Friday.”
keep something under your hat is a phrase.
Also accepted in games: keep it under your hat
Practised in Idioms