Am wondering if "Janus idioms" are a thing, like how Janus words mean themselves & their opposite (cleave, overlook, dust, ambivalent)

Eg: "a few bad apples", "blood is thicker than water", "exception proves the rule" which are often used to mean the opposite of original saying

"A few bad apples" is often used to mean that a few isolated bad individuals shouldn't be used to write off all of a group as bad, but the full saying is "a few bad apples can spoil the crop", referring to how rot will spread from one apple to another.

I has thought it was just that we don't need to worry about long term apple storage anymore because of refrigeration, but we could apparently pin the flip moment for this one to a 70s hit song "One Bad Apple (Don't Spoil the Whole Bunch, Girl)"

"Exception proves the rule" is one of my favourites because it hinges on the shifting meaning of "prove" and how it's meaning flipped.

The phrase refers to "prove" in the archaic sense, meaning "to test" rather than "means is true".

So a phrase that originally meant (though this is itself contested) exceptions were a challenge to a rule, that they require the rule to prove itself.

"Blood is thicker than water" is used to mean you should be more loyal to ppl biologically related to you.

It is often said to be a misquote of "the blood of the covenant is thicker than the water of the womb" (meaning the opposite), but that itself might be a 20thC fabrication.

"Curiosity killed the cat" is another good one, originally "care killed the cat" when used by Ben Johnson & Shakespeare "What, courage, man! what though care killed a cat, thou hast mettle enough in thee to kill care!"

With the archaic meaning of care, ie. worry & anxiety.