Everyone is fascinated by words like disgruntled and unkempt that have no corresponding unprefixed forms; aldiboronti, in a Wordorigins thread on the subject, links to an excellent World Wide Words post that goes into the history of the best-known such words with admirable thoroughness. On unkempt, for instance:
The word unkempt has a complicated history. Kempt comes from the Old English word kemb, “comb”. It seems to have gone out of use about 1600 but to have been reintroduced about 1860. Its usual and literal negative form was unkembed which survived into the middle of the nineteenth century. The form unkempt began to be used about 1580 to mean “language that was inelegant or unrefined”. In the eighteenth century it came to mean specifically “uncombed; dishevelled”, perhaps influenced by the Flemish equivalent ongekempt, and was used alongside the older form for about a century, only taking on a stronger sense of “neglected; not cared for” in the middle of the nineteenth century. Incidentally, the root form of kemb seems to come from a Germanic form which meant “tooth”, so a comb is named for its teeth; the modern form uncombed appeared about 1560.