IDIOMS: Let the Cat out of the Bag
Example: Carol's little brother the cat out of the bag about her surprise party.
Meaning: to give away a secret.
Origin: Centuries ago in England you might have bought a costly pig at a farmer's market. But if the merchant was dishonest and put a worthless cat into the bag instead of a piglet, you might not find out until you got home and let the cat out of the bag.
Source: Scholastic dictionary of idioms: over 600 sayings, phrases and expressions / Marvin Terban. 1996.