История реле: просто соединить

От названия телефонного разъема "jack-knife switch" в ранних коммутаторах. Название разъема — от jackknife (складной нож) "Origin 1705-15, Americanism; jack 1 (cf. jockteleg) + knife".

Заметка про "jack": https://www.theatlantic.com/technology/archive/2011/09/why-do-we-call-it-a-telephone-jack/245892/ — Why Do We Call It a Telephone 'Jack'? — ALEXIS C. MADRIGAL SEP 29, 2011:

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 (US224653 A — 1879) 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."

Другой патент 1880 года — https://www.google.com/patents/US489570 "US489570 A. SPRING JACK SWITCH.", C. E. SCRIBNER — https://patentimages.storage.googleapis.com/pages/US489570-0.png

Еще заметка про jack: https://mashedradish.com/2016/09/09/why-do-we-call-them-headphone-jacks/ "Since at least the late 1300s, jack has been naming all sorts of mechanical devices. One prominent contrivance is the Jack of the clock, simply called Jack at the end of 1400s. This was a little, mechanized man who strikes the bells on old clocks.… Early on, many of these jacks replaced the work of a man.… And so the various tools and technologies took the name of the man they stood in for: Jack."