open a can of worms

Example: “Asking about the inheritance opened a can of worms in the family.”

Origin: Fishermen once bought earthworms for bait pre-packaged in sealed metal cans, rather than in the plastic tubs or foam containers sold today — a genuinely literal can, genuinely full of worms. The trouble starts the moment the lid comes off: live worms immediately begin trying to escape, and getting them back into the can once they've started crawling out is close to impossible, a small, self-inflicted mess that only gets worse the more you try to fix it. The figurative sense, opening up a complicated, messy situation better left alone, is first documented in a 1951 Michigan newspaper, with the Oxford English Dictionary's own earliest print citation coming later, from a 1962 Times article.