RE:
Where did “therein lies the rub” come from? Is it misquoted from Hamlet or did it originate elsewhere?

This quote is from memory, but it is close enough.

Shakespeare is famous as an originator of original expressions that have become clichés, and this is an example.

” ‘Devoutly to be wish’d. To die, to sleep;

To sleep: perchance to dream: ay, there’s the rub:

For in that sleep of death what dreams may come,

When we have shuffled off this mortal coil,

Must give us pause: there’s the respect

That makes calamity of so long life;’

“The phrase uses “rub” in its less common definition as “obstacle” or “snag” rather than the more common usage of applying pressure to a surface”