Idioms are fascinating. They can be simultaneously completely understood and completely incomprehensible. Since it is usually so hard to connect the actual words to the general meaning we assign to the idiom. A literal interpretation of most of our common English idioms (or idioms in any other language) would reveal nonsense. What in the world do beans have to do with a secret? And why should ‘to spill the beans’ mean to reveal unknown information? Why shouldn’t we say that “dumping the figs” or “cracking the nut” means the same thing? Even more revealing is the expression with similar meaning: To let the cat out of the bag. We also need to ask whether there was a time when idioms such as these did not have mysterious origins. That is, whether they had a more transparent meaning.
See also: Origin of Egg on Your Face Expression
All we have to do to answer the last question is to consider common idioms of current origin. We know what we mean when we say someone has “gone postal” and we know that, generally, the expression derives from incidences involving postal workers. Although this is an unfortunate slur against postal workers, in that they are not more prone to violence, murder, and suicide than members of any other workforce, the idiom is fairly transparent to us. Although there are some idiomatic expressions that seem to have no literal interpretation at all, surely, many other idioms that have their origins in the distant past also, at one time, were more obvious to their users.
Before I try to give the possible origin of the idiom to spill the beans, I should point out that these two similar expressions, to spill the beans and to let the cat out of the bag, do not actually mean the exact same thing. That is, the meaning of an idiom has to do with the circumstances in which we will use one idiom instead of another with a similar meaning. Although many people use both these interchangeably, it is commonly accepted, and revealed in usage, that they have different shades of meaning and are used in different circumstances.
Once we realize this, we can consider that, if idioms usually have obvious origins when they first come about, they also refer to something fairly specific, so that in the idiom “to spill the beans” neither the act of spilling, or the beans themselves, are arbitrary choices. Given that, not all such idioms have this problem. If you “hit the nail on the head” it means you have done, or said, exactly the right thing and you have been very effective. It isn’t a stretch at all to see how striking a nail precisely on its head, in order to drive it in, would mean what it means in this expression.
When we say someone “spilled the beans” we usually mean that there is a select group of people who are privy to some information, and those people do not wish that information to be revealed to anyone else. If one member of this select group does reveal this information, usually by just blurting it out without thinking, so that an outsider, or the public at large, learns of the secret, then the secret teller has spilled the beans. This usually applies to the kind of information that not only do we wish to keep secret but if that secret were revealed the consequences would be more than just spoiling a surprise! They would tend to be negative consequences. On the other hand, to let the cat out of the bag can mean the same thing, but it usually refers to secret information that is not necessarily harmful if revealed, to the secret keepers or the people who find out. If you are planning a surprise party for someone and someone else involved in the planning inadvertently lets the subject of the party know about the surprise, they have let the cat out of the bag. The surprise is spoiled, but nobody will be incarcerated!
Most people think that they understand, to some extent, how to spill the beans means to let secret information slip. We can imagine that if beans were in some container, say a sack, and we spilled them out onto a table, we have revealed what was in the sack. So we partially understand it, so we think. But why beans? Why would beans be secret?
One possible origin of the spilling the beans idiom is that it originated as part of a game that was played at rural fairs in America. In this game, contestants would guess the number of beans in a jar. You probably know of similar games, where, as a child, you guessed how many of an object a big jar held, possibly pieces of candy. After all the bets were placed and the guesses made, the beans would be spilled out of the jar to reveal how many beans there were. So, if you asked someone to spill the beans, you were asking them to reveal some sort of secret information. It is probably obvious that when this expression came about, it was used both literally, as during the game, and figuratively, and in the expression, but over time, the game ceased to be played, while the expression stayed, loosely used to refer to the revealing of any type of secret info.
Even if the game is not the true origin of the idiom, it probably has a similar origin. For instance, another claimed origin is that it comes by way of the Ancient Greeks, who would vote at public elections by putting beans into a jar. There was a “YES” jar and a “NO” jar. When all the votes were in, the beans would be spilled out of the jar to reveal the number of yes beans and no beans.
It seems difficult to fathom how the expression could have not only survived but found it’s way from Ancient Greece to modern English, even though translations, especially if this practice of using beans to vote did not extend past Greece. Also, there is another expression that comes from a game and has to do with revealing information. The literal origin has remained obvious to us since the game has not only survived but thrived: Putting your cards on the table.
Still, we could ask whether “to spill” has had a long-term association with the divulgence of information. And this expression is convenient in that its components are idiomatically combined. The beans are information and to spill is to reveal or divulge. We may not be able to readily see how beans became information, but we can see that there are other expressions using the verb spill that have to do with revealing information. We might ask someone to spill it if we want them to tell the truth or reveal a secret. As well, to “spill your guts” means to tell the truth or give secret information. These expressions often have the further connotation of being a confession. Although the expression “spill the beans” did not appear in print until 1919, the word spill has been used to mean divulge since the 1500s.