In English, the prefixes in and un, along with others, are used to negate certain root words. The antonym of sane, for example, is insane, and the antonym of stoppable, for example, is unstoppable. But some words that appear to be negated with prefixes have no positive. The opposite of inept is not ept -- there is no such word. 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 Words With Uncommon Positive Forms The following words do have positives, formed by removing the negating prefix, but the positive forms of these words are far less common: disarray disconcerting immaculate impeccable inadvertent incapacitated incorrigible inevitable innocent inscrutable insensate insufferable interminable unbridled unflappable unfurl unkempt unmitigated unrequited unruly unthinkable unwieldy

Suffixes

Sometimes antonyms are formed by appending the suffixes -ful and -less onto English words, as with joyful and joyless. But sometimes one is an English word and the other isn't. For example, reckless is an English word, and reckful is not. Other times, one is more commonly used than the other: ruthless and ruthful are both legitimate English words, but the former is used far more frequently than the latter.

Special Cases

An interesting special case is indefatigable, whose antonym is not defatigable but rather fatigable.

The antonym of the verb incline is disincline, which is arguably not a double negative but in any case looks like one.

The word inflammable is another interesting special case. Stripping off the prefix does not create an antonym but rather a synonym: flammable. This idiosyncrasy of English seems insidious (!) until one realizes that the prefix in- means something else in this case. The word inflammable means "able to be inflamed." Now it makes sense, doesn't it?