I stumbled across the answer to this question in James Gleick's spectacular book, The Information. Here's Gleick's capsule history:
George W. Coy, a telegraph man in New Haven, Connecticut, built the first "switch-board" there, complete with "switch-pins" and "switch-plugs" made from carriage bolts and wire from discarded bustles. He patented it and served as the world's first telephone "operator." With all the making and breaking of connections, switch-pins wore out quickly. An early improvement was a hinged two-inch plate resembling a jackknife: the "jack-knife switch," or as it was soon called, the "jack."The patent is now available online. It begins, "Be it known that I, George W. Coy, of New Haven, in the county of New Haven and state of Connecticut, have invented a new Improvement in Electric Switches." But the jack-knife switch itself was not invented by Coy. That honor goes to Charles Scribner of the Western Electric Company.
We love our tech etymology.
This article available online at: