keep under hat
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Related to keep under hat: keep under one's 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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
keep (something) under one's hat, to
To preserve secrecy. This term, with its vivid image of hiding a secret under headgear, dates from the late nineteenth century and has remained current even though hats are worn much less frequently today. A popular song of the 1920s had it: “Keep it under your hat! You must agree to do that. Promise not to breathe a word” (quoted by Eric Partridge). See also in the dark.
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer