As transport in the 1800s, balloons had a frustrating drawback: they were at the mercy of the wind. With the invention of lightweight engines, it became possible to direct the course of a balloon, a distinct improvement. The word “direct” comes from the Latin dirigere, to straighten, set straight, direct, guide. Thus “dirigible balloon”: a balloon that can be guided, or directed, by its pilot, instead of being passively directed by the breeze.
(And soon enough the adjective “dirigible” was nouned into a balloon.)
I’m not suggesting that the etymology of a word like “dirigible” is an authoritative guide to its meaning; it is not. Words grow up and leave home. Some make bad choices, but we love them anyway. So be it.
Still, etymology enriches our experience of our language, dirigible being a case in point. Thus Alban Joseph Zolly, Mr Zolly to us eighth-grade English students at Camp Zama in the early 1960s (I incant his full name in the slim hope that someone might someday search the web for his name and turn up this remembrance).
Dirigible/direct is part of a pattern that includes corrigible/correct, negligible/neglect, intelligible/intellect and eligible/elect, among a few others (when’s the last time you heard “erigible”?). We have legible, but “lect” survives only in works like lector, lectern, lecture. Patterns have a mysterious power to explain, though in a sense they only deepen the mystery.
Of the list, “eligible” caught my eye, I suppose because of my side interest in elections and voting. In English, perhaps especially American English, “elect” is highly associated with voting. But its meaning is rooted in the idea of “choose”, and a moment’s reflection will remind us that we still use it that way from time to time, and that the voting sense is a fairly obvious outgrowth of “choose”: we choose our leaders, representatives, whatever. But we still elect to do something, take elective courses, and the like. So to be “eligible” is to be capable of being chosen, or elected.