Why do we say this when a secret is revealed?

Sources used:

• Oxford English Dictionary (OED) – historical citations for “let the cat out of the bag” and idiom notes

• Merriam-Webster Dictionary – idiom entry and usage history

• Etymonline (Online Etymology Dictionary) – discussion of proposed origins and early attestations

• The Phrase Finder – idiom background, “pig in a poke” connection

• Smithsonian Magazine (language/etymology features) – context on idioms with uncertain origins