I think it's an interesting word too. I never thought of looking into the etymology because in Spanish to me when I think of francotirador, I think it refers to the ability to aim well, es decir, la punter?a. Therefore, from that perspective if you have a good aim, you are sharp or direct, and you will hit your target. In other words, a sharp shooter.
I read Wikipedia's description last night when I saw the thread but under the meaning of francotirador and not the French word. The explanation is quite good.
-MP.