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Difference between Have something under one’s hat and Keep something under one’s hat

have something under one’s hat—(coll.) have knowledge of smth.:

  • He had something under his hat; a tip-off straight from headquarters, news of high international importance.

keep something under one’s hat—(coll.) keep smth. a secret; keep smth. to oneself:

  • “Nothing about that in the papers, is there?” “Not yet. Keep it under your hat.”

See also: pick something out of a hat / pull something out of a hat.