Unkempt is one of those words for which there isn’t a simple antonym. Removing the ‘un’ leaves us with ‘kempt’ which certainly doesn’t mean neat or tidy, at least not to the modern English speaker.
It is one of those words where the negative remains current long after the negative has faded from speech. Kempt once meant ‘well-combed’, indeed the word comb derives from the Old English verb ‘kemb’, so I suppose the answer to 18 down should have been ‘uncombed’ in any logical language.
I’ve been meaning to make a list of such words as they occur to me, but I haven’t because of my fecklessness. Oh, there’s another one – my lack of purpose or sense of responsibility might make me feckless, but had I been more diligent you wouldn’t think I was ‘feck’, although if you say ‘effective’ quickly you might get a clue as to the derivation of the older word.
Less obvious is the word ‘reckless’ from the Old English ‘receleas’ meaning thoughtless or careless. Our forebears once had ‘reccan’ to describe taking care or heed, yet we don’t have that luxury. J R R Tolkein used it in Return of the King in 1955, but unearthing it elsewhere requires some reccanful literary archeology.
Then there are the ‘dis-’ words like disgruntled which literally means being subject to frequent gruntling. The gruntle in this case is to grumble, rather than making some sort of animal noise, although I suppose it could be easy to mistake the sound of one for that of the other.
Finally, we have ruthless. We seldom, if ever, describe a compassionate person as ‘ruthful’ in the way we might once have done. Which is a pity – or a ruth if you prefer.
There must be lots of others that I haven’t thought of so please feel free to add your contributions.