In sports jargon, the second string is the second-best group of players on a given team.
The term has also found its way into business, where it is used in much the same way.
In fact, it comes from medieval archers, who always carried an extra string in case the one on their bow broke.
Therefore the second string had to be as good as the first, as did the third and fourth strings.
I believe that the this saying arose because, the archer also carried shorter arrows and in his string packet a few shorter strings. If the bow broke he would revert to this more ancient way of delivering the arrow before having to revert to the mall and dagger he also carried.
Second String!
I know that the archer carried shorter arrows and that he also carried smaller goose greased strings. I read it in some ancient well respected document (book) somewhere, which I am still trying to relocate.
It is hand thrown, as the Danish, Yorkshire & Devon bung Arrow. These were weapons in their own right and predated the bow and arrow by millennia.