I'm Not Disappointed, I'm Very Appointed!

Your dad says that sometimes...
They're called unpaired negatives, those words that (apparently) have no opposite.

I have no illusions about my ability to explain this... I hope you won't be disillusioned.

Way back when, mayhap you will recall that hap was related to happen and was the positive
that corresponded to hapless.

If you read stories about the age of chivalry, you will remember that knights wielded swords with great skill, and wield was the positive that corresponded to unwieldy.

Though wordsmiths may use somnolent to correspond to insomnia
In modern English there is no
gainly corresponding to ungainly, nor
gruntled to correspond to disgruntled or
pulsive for repulsive

In many other cases, the positives that existed in Old English have disappeared, leaving only the negatives

For a more complete discussion (and the origin of unkempt) see Wide World of Words
http://www.worldwidewords.org/articles/unpaired.htm

also a brain-teasing poem using unpaired negatives
http://blogs.abc.net.au/queensland/2008/03/daniel-viless-v.html