Meaning: Give away a secret
While also one of the best preschool-era games ever, spilling the beans is believed to have originated in ancient Greece. Back then when there was an issue that needed to be decided (e.g. is tonight a taco or Sloppy Joe night?) a vote was held. White beans were a "yes" and black beans were a "no" vote. If the vote collector dropped the jar and a black bean was seen, the vote was ruined.