What led me to this unfortunate discovery was a student, predictably, who had asked me why "ruthless" was a word but there was no word for its opposite condition ("ruthful," presumably?). I didn't know, but it did put me in mind of the following couplet:
We rode in my convertible, my girlfriend Ruth and me,So I went to look it up. It turns out that the "ruth" in "ruthless" is a cognate of "to rue," meaning "to afflict with contrition or sorrow." So "ruthless" originally meant "lacking contrition." The word "rue" only remains in English in the construct "to rue the day," as in, "you'll rue the day you ever double-crossed me, you dastardly and uncouth villain!"
I hit a bump doing 95, and I went on, ruthlessly.
Which brings us to "uncouth." There's no such word as "couth," however people joke about it. The current meaning of "uncouth" as "wild-looking, dirty, scary," is because the last part of the word comes from the Indo-European root "kynths," meaning "known." So "uncouth" really means -- and is a cognate to -- "unknown," not "unkempt" (whose meaning it resembles more closely today). And, by the way, the "kempt" part of "unkempt" comes from Old Norse, "kembr," meaning "combed." So as that goes, "unkempt" and "dishevelled" were cousins a millenium ago, and still are; "shevelled" comes from Old French "chevel," meaning "hair." Both, essentially, meant "having a bad hair day," a narrower meaning than today, when both of them usually simply mean "untidy, rumpled-looking." (And in case you are wondering, I am both kempt and shevelled today, not to mention highly couth.)
"Disgruntled" is kind of a funny one, because here "dis" is not used in its most common meaning of a negative, but in its far less frequent role of an intensifier -- the only other example I could find was the obscure "disannul." The "gruntled" part is a cognate of "to grunt" in its old sense of "to complain." So really, it means "feeling like complaining really loudly." But it's a pity that it's not one of the opposite-words, like the previous examples. I think that having "gruntled" mean "cheerful" would just be wonderful.
"Nonchalant," and its noun form "nonchalance," are predictably from French, and were only adopted into English in the 18th century. The last part of the words comes from "chaloir," meaning "to worry, to be concerned with," so "nonchalant" hasn't changed much in meaning since that time. Still, you have to wonder why we can't be "chalant." I certainly am, sometimes.
A lot of "mis" words have no opposites. You can be a misanthrope, but not an anthrope; a miscreant but not a creant; you can commit a misdemeanor, but not a demeanor. A mishap occurs when you are unlucky, but only the hapless among us would describe winning the lottery as a "hap."
So anyway, you get the picture. As usual, the answer to my student's question about why such things happen in languages was "damned if I know." I doubt much of this was new to you -- probably most of these examples were both toward and heard-of -- but perhaps you had never really stopped to think about the question before, so I hope this post was called-for, and that you were able to make both heads and tails out of it.