An impediment. The expression is taken from bowls, where “rub” means that something hinders the free movement of your bowl.
“Without rub or interruption.” —Swift.
Like a bowle that runneth in a smooth allie. Without anie rub.
—Staniharst, p. 10.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894