die aap uit die mou laat

From older die aap kom uit die mou (“a former secret becomes publicly known”), but influenced (both syntactically and semantically) by English let the cat out of the bag, which has also been calqued as die kat uit die sak laat.

die aap uit die mou laat

1. (idiomatic) To let the cat out of the bag; to disclose a secret; to let a secret be known, often inadvertently.

   Synonym: die kat uit die sak laat

   Ek het die aap uit [my] mou gelaat.

   I let the cat out of the bag.

   (literally, “I let the monkey out of my sleeve.”)