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;
- โkeep it under our hatโ 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 Mayorโs 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 classesโฆ
Joey: Which we'
re still keeping under our hats!