From Longman Dictionary of Contemporary Englishlet sliplet slipto accidentally tell someone something that should have been kept secretlet slip that Liz let slip that she’d seen him quite recently. → let
Examples from the Corpus
let slip• The chance to convert a proposal long mooted into reality was too precious to let slip.• It was true that Miss Devenish let slip a good deal of fragmentary information about Tolby.• He had let slip he was married.• I happened to let slip how much James gave me for it.• Not a word had Dooley let slip, not even a hint.• On the few occasions I have met him it has been very difficult for me not to let slip something about you.• Even if he did not allow himself to betray his secret directly he might let slip something that would provide a clue.• Alex let slip that he had spoken to Julie on the phone and knew where she was.• He inadvertently let slip the name of their new product.• From there on, they never let slip their icy grip on the game. 