the.com/rub

means To press and move something against a surface with friction — and, in the old idiom, the hidden snag or difficulty in a situation.

from From Middle English 'rubben,' of uncertain origin — likely related to Low German and Dutch words for rubbing or scraping, though the deeper roots are murky. The figurative sense ('there's the rub') comes from a now-quaint use in the game of bowls, where a rub was an obstacle on the green that knocked a ball off its course — Shakespeare made it immortal in Hamlet's 'to sleep, perchance to dream — ay, there's the rub.'