Spill the beans
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The phrase Spill the Beans means to reveal a secret, usually accidentally. It also implies that the revealed secret ruins a surprise or plan of some sort.
The origin of Spill the Beans is not clear, although the word 'spill' can be used alone or with other words to mean the same thing, as in 'spill your guts'.
A popular belief as to the origin of the phrase dates back to ancient Greece, when existing members of secret societies voted upon applicants by placing either a black bean or a white bean in a jar. If the jar was knocked over before the conclusion of the ceremony, the results would be prematurely revealed. This theory is not possible, however, as the phrase dates back only to the early part of the twentieth century in the United States.[citation needed]
(Taken from [1])