A pair of examples: “bought a pig in a poke” and “let the cat out of the bag.” “Poke” is an archaic term for “bag” [0], related to “pocket.”

Apparently a common medieval scam was to try to sell a cat as a suckling pig, concealing the cat in a bag [1]. One who buys such a pig in a poke is cheated. One who lets the cat out of the bag reveals a secret too soon.

Note however that [2] argues that this etymology for lack out of the bag” is not plausible.

[0] Etymology 2, https://en.m.wiktionary.org/wiki/poke

[1] https://en.m.wikipedia.org/wiki/Pig_in_a_poke

[2] https://www.snopes.com/fact-check/let-the-cat-out-of-the-bag..., referenced from https://en.m.wikipedia.org/wiki/Letting_the_cat_out_of_the_b...