What do you call words that look like a negation but are not?

I can be nonplussed (in fact that is practically the ground state of my existence), but not plussed. I can also be indifferent; but if you are different, that doesn't mean you care, either. What do you call it when a word looks like the negation of another word but is not? The kind of words that Terry Pratchett takes delight in?

(possible 2nd question) I had started by looking up wittingly, seeing that I've never seen it used except in the phrase "wittingly or unwittingly", or ironically, as the opposite of unwittingly. There might even be another class of word like "wittingly", words that are only ever used with their opposite.

Solution 1:

It seems they are called Unpaired Words (maybe the best definition) or Absent Antonyms.

Unpaired Words
An unpaired word is one that, according to the usual rules of the language, would appear to have a related word but does not. Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite. Many unpaired words are the result of one of the words disappearing from popular usage, though others were never part of a pairing and just begin with the same letters as used in common prefixes. The classification of a word as “unpaired” can be problematic, as a word thought to be unattested might reappear in real-world usage

You can find some examples here, but I'll list the majority of them here anyway for easy reference.

1. Words with no positive forms:
   Debunk; defenestrate; dejected; disconsolate; disdain; disgruntled; dishevelled; dismayed; disrupt; feckless; gormless; impetuous; impromptu; inane; incessant; inchoate; incognito; incommunicado; indomitable; ineffable; inept; inert; infernal; inhibited; insidious; insipid; insouciant; intact; invert; misgivings; misnomer; nonchalant; noncommittal; nondescript; nonpareil; nonplussed; unbeknownst; ungainly; unswerving; untold; untoward.

2. Words with uncommon positive forms:
   Disarray; disconcerting; immaculate; impeccable; inadvertent; incapacitated; incorrigible; inevitable; innocent; inscrutable; insensate; insufferable; interminable; unbridled; unflappable; unfurl; unkempt; unmitigated; unrequited; unruly; unthinkable; unwieldy.

3. Suffixes (asterisk means "word not existing"):

   • Reckless/*Reckful
   • Indefatigable/*defatigable -> fatigable
   • Flammable-Inflammable (not antonyms)

Solution 2:

I'm not certain what to call them, unless you accept loanwords as the answer. Of course these aren't just any loanwords; these are loanwords from languages that share cognate roots with English. For example, nonplussed and its noun form nonplus come from the Latin words nōn and plūs, meaning "no" and "more"; English has incorporated both non- and nonplus, but not plus. (At least in the sense of the anti-negation of nonplus -- of course English did incorporate plus, but separately, so that the two are unrelated.) So nonplus is the negation of plus, but only in another language!

I'll add that, as Alenanno has demonstrated, not all words with this property are loanwords.