Recently I had occasion to use the word unsaid, as in what goes unsaid. Looking at that phrase later, I began to ponder the related verb unsay, which means something different. What is unsaid is not said, but to unsay something means to retract it. The same not-quite-parallelism holds for unseen and unsee and unheard and unhear. Sometimes un- means not and sometimes it means to reverse.
The pattern, as linguists will tell you, has to do with using a word as a verb versus using it as an adjective. To un– a verb is to reverse the action of something: to undress, untie, unzip, unfold, unpack, untuck, untwist, unroll, unveil, unwrap, undo, and many more. Adding un– to a verb was a favorite trick of Shakespeare’s yielding such words as to unsex, to uncurse, and to unshout.
To un- an adjective is to negate the quality described by the adjective: unabridged, unacceptable, unanswered, unbalanced, uncommon, unlucky, untidy, untrue, unwritten, and so on. Some of these adjectives are just un- plus a straight-up adjective—acceptable, common, lucky, tidy, true. Others are made from the past participle of the related verb: abridged, answered, balanced, written. In each case, the meaning is “not” rather than “reverse.” An unabridged dictionary is not one in which words have been put back in, but one in which they are not left out. An unanswered question is not one receiving a bogus answer, but one getting no answer.
The story of un- gets tricky though because sometimes past participles serve as verbs, which allows ambiguity: The box was unpacked. The baby was undressed. The jacket was unzipped. The gift was unwrapped.
Each of these has an adjectival sense, in which the box was not packed, the baby was not dressed, the jacket was not zipped, the gift not wrapped. But each also has a reversed sense in which some unnamed person is unpacking the box, undressing the baby, unzipping the jacket, or unwrapping the gift. Of course, sometimes only one meaning is possible, as in (the classic example) Antarctica is uninhabited, which cannot mean that someone is uninhabiting Antarctica.
The Oxford English Dictionary 2018 update gives nearly 300 un- plus adjective combination, including unadult, unblasé, unsorry, and un-with-it. Nouns with un- are usually derived from adjectives, so they carry the sense of not rather than reversal: uneasiness, untruth, and so on.
Curiously, in a handful of words un– seems unnecessary but shows up anyway. The most widely used is unloose/unloosen, which the OED attests as early as the fourteenth century. Perhaps analogy with other un-verbs (untie, unfasten, unleash) is a factor in the unnoticed redundancy. Unthaw, meaning to thaw out, is attested as early as 1700, and today may even be heard in your own kitchen.
Unravelling unravel is trickier. Ravel it turns out is a contranym: a word which can mean either entangle or disentangle. So the un- of unravel does some work here, disentangling the senses of the root.
And finally, some instances of un- are mere historical vestiges. Uncouth and unkempt began as the prefixed words uncūth meaning “unfamiliar” and unkemd meaning “uncombed.” As the meanings shifted, the roots cūth and kemd became obscured and today we longer view uncouth and unkempt as prefixed forms at all.
So the next time you use an un- word, pause for a second and mentally take the word apart. You may find the experience uncanny.