Learn English – ny word, alternative to the idiom, “rack one’s brain”

The Oxford Dictionary Online (now Oxford Languages) says that the phrase could use either wrack or rack. They note that

The relationship between the forms rack and wrack is complicated. The most common noun sense of rack, ‘a framework for holding and storing things’, is always spelled rack, never wrack. In the phrase rack something up the word is also always spelled rack. Figurative senses of the verb, deriving from the type of torture in which someone is stretched on a rack, can, however , be spelled either rack or wrack: thus racked with guilt; or wracked with guilt; rack your brains; or wrack your brains.

However, according to this entry for wrack in EtymOnline, the term should be rack:

The verb meaning "to ruin or wreck" (originally of ships) is recorded from 1560s, from earlier intrans. sense "to be shipwrecked" (late 15c.). Often confused in this sense since 16c. with rack (1) in the verb sense of "to torture on the rack;" to wrack one's brains is thus erroneous.

The PhraseFinder agrees that the phrase is rack your brains, adding:

The rack was a mediaeval torture device. The crude but, one presumes, effective racks often tore the victim's limbs from their bodies. It isn't surprising that 'rack' was adopted as a verb meaning to cause pain and anguish. Shakespeare was one of many authors who used this.

Further, this book on common English errors says it should be rack:

If you are racked with pain or you feel nerve-racked, you are feeling as if you were being stretched on that Medieval instrument of torture, the rack. You rack your brains when you stretch them vigorously to search out the truth like a torturer. “Wrack” has to do with ruinous accidents, so if the stock market is wracked by rumors of imminent recession, it’s wrecked. If things are wrecked, they go to “wrack and ruin.”

The Grammarist agrees it should be rack as well.

That being said, there is some use of wrack your brains (blue line) as shown by this Google NGram. However, rack your brains is correct and more common:

There is some disagreement, but more sources say it should be rack. Thus it seems more likely that the phrase is rack your brains instead of wrack.

Not knowledge per se, but the classic warning against the pursuit of knowledge is curiosity killed the cat:

Wikipedia:

“Curiosity killed the cat” is a proverb used to warn of the dangers of unnecessary investigation or experimentation. …

The Phrase Finder:

Inquisitiveness can lead one into dangerous situations.

The Free Dictionary:

Prov. Being curious can get you into trouble.  (Often used to warn someone against prying into other’s affairs.)
• Jill: Where did you get all that money?
  Jane: Curiosity killed the cat.

the Cambridge English Dictionary:

said to warn someone not to ask too many questions about something

I could tell you, but then I would have to kill you.

Quora:

I could tell you, but then I’d have to kill you has become a way to let someone know they’re asking you something you don’t want to answer (and a rather colorful way of telling them to mind their own business!).

This line comes from "The Hounds of Baskervilles" featuring Sherlock Holmes.  Here is the exchange:

Sherlock: I never did ask, Dr. Franklyn.  What is it exactly that you do here?
Doctor: Oh, Mr. Holmes, I would love to tell you, but then, of course, I’d have to kill you.
Sherlock: That would be tremendously ambitious of you.

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The line has since appeared in many movies, including Top Gun with Tom Cruise.  Here’s a handy YouTube video compiling many snippets with this line and variations on it.

The Free Dictionary:

a phrase said in answer to a question that one does not want to answer.  Don’t ask.

TV Tropes:

Often heard in settings related to espionage and high security levels, the phrase “I’d tell you, but then I’d have to kill you” is itself probably a Dead Horse Trope by this point – whether for a serious straight use or not.

As the quotes above indicate, this has become something of a joke.

While not exactly an idiom or proverb, the phrase knew too much is evocative of the films The Man Who Knew Too Much, in which a man is murdered for learning of a criminal conspiracy.  References: [IMDb 1934], [IMDb 1956],  [Wikipedia 1934], [Wikipedia 1956].

There’s a well-known proverb, “A little learning is a dangerous thing,” often misquoted as “A little knowledge is a dangerous thing.”  For years I (mis)understood this to mean that knowledge is dangerous — so dangerous, so potent, that it is dangerous even in small quantities.  But when one sees the complete sentence, which appears in Part 2 of An Essay on Criticism by Alexander Pope:

A little learning is a dang’rous thing;
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.

it becomes clear that the message is that partial or incomplete knowledge is dangerous, while thorough or complete knowledge is a good thing.

The Phrase Finder:

A small amount of knowledge can mislead people into thinking that they are more expert than they really are.

Dictionary.com:

Knowing a little about something tempts one to overestimate one’s abilities.

and from this we get the phrase (again, not exactly an idiom or proverb) knowing just enough to be dangerous.