This is a very crude synonym to the more technical term, masturbation. Once masturbation is brought to mind, you should understand the imagery that this term conveys. If you still don't get it, think male masturbation, and think of the "stone" as two stones instead of one.
Well, that was a long stressful day... I think it's time to relieve some stress, maybe an afternoon of trying to pull the sword from the stone will help.