under one's hat

Prepositional phrase

idiomatic

(idiomatic) Concealed; confidential; secret.

Japanese Meaning

秘密に / 内密に / 隠しておく

Please keep this plan under one's hat until we finalize the details.

詳細が確定するまで、この計画は内緒にしておいてください。

Sense（1）

(idiomatic) Concealed; confidential; secret.

Orestes Jones kept his real ambitions under his hat until the time struck.