The Collaborative International Dictionary of English v.0.48:

Rub \Rub\, n. [Cf. W. rhwb. See Rub, v,t,]
   1. The act of rubbing; friction.
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   2. That which rubs; that which tends to hinder or obstruct
      motion or progress; hindrance; obstruction, an impediment;
      especially, a difficulty or obstruction hard to overcome;
      a pinch.
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            Every rub is smoothed on our way.     --Shak.
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            To sleep, perchance to dream; ay, there's the rub.
                                                  --Shak.
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            Upon this rub, the English ambassadors thought fit
            to demur.                             --Hayward.
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            One knows not, certainly, what other rubs might have
            been ordained for us by a wise Providence. --W.
                                                  Besant.
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   3. Inequality of surface, as of the ground in the game of
      bowls; unevenness. --Shak.
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   4. Something grating to the feelings; sarcasm; joke; as, a
      hard rub.
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   5. Imperfection; failing; fault. [Obs.] --Beau. & Fl.
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   6. A chance. [Obs.]
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            Flight shall leave no Greek a rub.    --Chapman.
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   7. A stone, commonly flat, used to sharpen cutting tools; a
      whetstone; -- called also rubstone.
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   Rub iron, an iron guard on a wagon body, against which a
      wheel rubs when cramped too much.

   Rub of the green (Golf), anything happening to a ball in
      motion, such as its being deflected or stopped by any
      agency outside the match, or by the fore caddie.
      [1913 Webster + Webster 1913 Suppl.]