keep under hat

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Related to keep under hat: keep under one's hat

keep something under one's hat

Fig. to keep something a secret; to keep something in one's mind (only). (If the secret stays under your hat, it stays in your mind. Note the use of but in the examples.) Keep this under your hat, but I'm getting married. I'm getting married, but keep it under your hat.
See also: hat, keep

keep something under your hat

to keep something secret I've got some interesting news, but you must promise to keep it under your hat for the moment.
See also: hat, keep