Fig.: to reveal someone's private information; to disclose a secret; to confess

An FBI informant pretended to be his accomplice and the spy spilled the beans on what he had done

Un informatore dell'FBI si è finto suo complice e la spia ha vuotato il sacco su quanto aveva fatto

The children were sworn to secrecy, but as soon as their father raised his voice, they spilled the beans

I bambini avevano giurato di mantenere il segreto, ma appena il padre ha fatto la voce grossa, hanno spiattellato tutto

A popular theory on the origin of this expression takes us back to ancient Greeks who held secret votes on the admission of new members into their secret societies. Voters would vote a candidate by dropping a white bean (for yes) or a black bean (for no) into a jar. The beans were then supposed to be counted in secret, but if someone knocked over the jar and spilled the beans, the secret vote would be revealed.