I heard someone on TV this morning use the verb "pick off". Now, when I hear this I always picture a sniper—not necessarily a modern sniper, he could as easily be using a bow and arrow—choosing targets one by one, perhaps selecting ("picking") the most critical targets while those around him try to overwhelm the enemy by massed fire. Blam, blam, blam, they flood the enemy ranks with largely unaimed fire, while with a single shot the sniper picks off a leading charger, an especially effective enemy officer or the operator of an important gun. Blam, blam, blam go the massed troops again, while (twang) goes down another critical enemy. The sniper is picking them off one by one.
Metaphorically, then, to "pick off" a number of items is to hit / eliminate / collect them one by one, with an added sense of careful selection. But where did it come from? Was sniping indeed the original meaning, or did that come from something else? Anyone have a guess?
With no research or first hand knowledge, my guess is that it comes from taking fruit from a tree or bush, which is to "pick". "Picking off", then, would be more selective, getting only the best fruit, and this would run parallel to sniping.
Well, duh! Thanks, Dick; the answer is obvious once you point it out. I must not have had my head on straight.