The 18th-century phrase “he let the cat out of the bag” was used very recently by Ghislaine Maxwell’s lawyers in their efforts to convince a judge to block the release of documents which contained “intimate, sensitive and personal” information about Ghislaine Maxwell which she had divulged in a deposition taken in April 2016 for a defamation case brought by Epstein’s accuser Virginia Guiffre.

Ghislaine Maxwell’s lawyers want to keep what she said secret because if it is disclosed it will work against in her current case in which he faces charges of helping Jeffrey Epstein to recruit an abuse underage girls between 1994 and 1997. She is in custody awaiting trial next year. She is watched around the clock by psychiatrists dressed as prison staff!

What the lawyers are saying is that if they disclose this information about Ghislaine Maxwell it would be giving away what should be secret. Ironically, the original saying dates back to the 18th-century when it referred to a market-day trick according to Dr Desmond Morris.
The saying orignated in a con trick. The seller of piglets would try and trick the buyer by putting a cat or cats inside a bag ostensibly containing the piglets. The buyer would think that they are buying piglets when in fact they were buying a cat. But if the cat wriggled around too much the seller had to release the cat which exposed the seller’s trick, hence the secret is exposed and “he let the cat out of the bag”.
It is interesting that the original saying involves and attempted fraud by a trickster or conman. It’s a nice touch because you wonder whether Ghislaine Maxwell’s lawyers are involved in a sort of legal con in protecting their client unreasonably. The truth should be out because it would assist justice. Maxwell’s lawyers say that justice would be obstructed if her earlier disposition was in the public domain. I disagree with them.