Let Slip Meaning

(idiomatic) To divulge a secret, as by accident or mistake.

Example:   He finally let slip that they plan to take over the business.
1911, Jack London, The Whale Tooth
  The frizzle-headed man-eaters were loath to leave their fleshpots so long as the harvest of human carcases was plentiful. Sometimes, when the harvest was too plentiful, they imposed on the missionaries by letting the word slip out that on such a day there would be a killing and a barbecue.