Q: What's a rub?
A1. I was always given to believe that a "rub" was a feature of a bowling green such as a ridge or hump that serves as an obstacle to make the game more difficult, now regularised in crown green bowls. Hence its use to mean an obstacle more generally.
A2. Shakespeare , Hamlet
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