Keep something a secret.

There are four origins of this idiom (I GUARANTEE IT):

- English archers protected their bows by putting the strings on their heads under their helmets;
- “keep it under our hat” was mentioned in 1982 in periodical "Gleanings in Bee Culture";
- President Abraham Lincoln's stove pipe hat, where he kept important papers;
- the ceremonial swordbearer of the Lord Mayor of London (can be dated to 1420), who keeps the key to the Lord Mayor’s seal of office in a special pocket in his hat.
-keep it under one's hat-

Joey: All right, what else?
Chandler: Well uh, there was acting classes, stage combat classes, tap classes…

Joey: Which we're still keeping under our hats!
by Kisa Boris July 13, 2016
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