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Idiom

Keep it Under Your Hat: Origin and Meaning

Meaning
Keep something secret, as opposed to spreading it around by telling everyone.
Origin
Keep your shirt onThis idiom means to keep a secret, the opposite of “let the cat out of the bag” (which means to reveal a secret).

There are two ideas about the origin of this idiom. One says it’s connected to the fact that archers in medieval Europe used to keep extra strings for their bows under their hats without anyone knowing about it. However, the phrase doesn’t appear until the 19th century – so it is unlikely to have a medieval origin.

The other idea says it’s connected to the fact that your head is under your hat, and that the idiom means to keep it in one’s head

An early example is found in P. G. Wodehouse’s “Inimitable Jeeves”, 1923:

It made such a hit with her when she found that I loved her for herself alone, despite her humble station, that she kept it under her hat. She meant to spring it on me later.

Examples
Keep this under your hat, but I’m getting married.

– Aunt Hyacinth said she couldn’t tell me the family secrets because she’d pledged (promised) on her mother’s grave to keep them under her hat.