What is "rub of the green"

Rub \Rub\, n. [Cf. W. rhwb. See Rub, v,t,]

1. The act of rubbing; friction.

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.

   Every rub is smoothed on our way.
   --Shak.

   To sleep, perchance to dream; ay, there's the rub.
   --Shak.

   Upon this rub, the English ambassadors thought fit to demur.
   --Hayward.

   One knows not, certainly, what other rubs might have been ordained for us by a wise Providence.
   --W. Besant.

3. Inequality of surface, as of the ground in the game of bowls; unevenness.
   --Shak.

4. Something grating to the feelings; sarcasm; joke; as, a hard rub.

5. Imperfection; failing; fault. [Obs.]
   --Beau. & Fl.

6. A chance. [Obs.]

   Flight shall leave no Greek a rub.
   --Chapman.

7. A stone, commonly flat, used to sharpen cutting tools; a whetstone; -- called also rubstone.

   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.

n. 1 (context golf English) Any accidental deflection of the ball by an outside agency (spectator, caddie etc); the ball is played without penalty from where it comes to rest 2 (context sports by extension English) an outside agent that affects the movement of the ball.