Uncle John knows pretty much everything—and if he doesn’t, he heads his massive research library, or puts one of his many associates on the case. So go ahead: In the comments below, ask Uncle John anything. (And if we answer your question sometime, we’ll send you a free book!)

How (and why) is “won’t” a shortening of “will not”? Where did that “o” come from…and where did the “ill” go?

A contraction is any word in which two other words are shortened, and then marked with an apostrophe to indicate that the splice has occurred. They reflect the way people talk, and are a much more colloquial way of presenting words, as opposed to the stiff way of writing out or saying both words. Some examples of contractions: “don’t” (short for do not) and “shouldn’t” (short for should not). In both cases, the apostrophe replaces an “o.”
Other contractions aren’t quite so straightforward. “Can’t” is a shortening and combination of can not, or the single word cannot. In this case, the apostrophe replaces the “o” and the redundant, second “n” is eliminated entirely. The strangest of all modern day English contractions is “won’t.” It’s a quicker way of saying “will not,” even though at first glance it seems like a completely arbitrary word surgery has taken place.
Won’t is a relic of a word that fell out of favor…or at least was corrupted over the centuries of a changing language. In Old English, wold was the past tense of the verb willan, or will. By the 16^th century, wold had changed to woll. Adding in “not” made the phrase woll not, contracted to “won’t,” because the double-ll was hard to pronounce and so disappeared from speech. The variant willn’t was used around the same time to mean the same thing, but that was hard to pronounce, too…and so it disappeared, leaving behind won’t.