The word "rub" in the context of Shakespeare's work and the time is synonymous with "problem" or "issue," so what it meant was - that is the source of the problem, or this is where the issue lies.
"Therein lies the rub" indicates that you have identified the root cause of the irritation. "That's what really ticks me off" is another way of putting it.
This phrase from Shakespeare's Hamlet means "therein lies the problem or dilemma".
He never returned to his home-town, and therein lies a mystery.
Excalibur's sheath was said to have magical abilities that no harm would befall he who wore it. Therein lies the rub, Arthur lost or forgot the original sheath and thereby went on to his doom.
Platypuses neither eat corndogs, nor are they kept as pets. Therein lies your problem.
Therein lies the problem with STD infections; you don't know for sure if the person is infected.
It surely does. But where is it, therin lies the rub.
white lies mean small lies<3
Literature unlocks the imagination and therein lies the ability to percieve the beauty and color that is life.
Check your local ordinances on livestock and therein lies your answer. Keep in mind that most municipalities will not allow it.