Wiktionary
let the cat out of the bag
vb. (context idiomatic English) To disclose a secret; to let a secret be known, often inadvertently.
WordNet
let the cat out of the bag
v. divulge confidential information or secrets; "Be careful--his secretary talks" [syn: spill the beans, talk, tattle, blab, peach, babble, sing, babble out, blab out] [ant: keep quiet]