Are you disgusted, disgruntled and disheveled? Well, unfortunately you’re never going to be gusted, gruntled or sheveled. Disgusted, disgruntled and disheveled are what you might call “lonely negatives.” They’re negative words whose positive partners have vanished or never existed in the first place.
(Via French or Italian, from Latin dis- ‘expressing reversal’ + gustāre ‘to taste.’)
English adopted only the negative version, leaving us without the useful expression, ”That gusts me.”
(From the late Middle English word, now obsolete, 'dishevely,' which derives from Old French deschevelé, past participle of descheveler, based on chevel, 'hair,' from Latin capillus. Originally it meant 'having the hair uncovered' and later it referred to the hair itself, hanging loose, and so messy or untidy.)
You can be disheveled without ever being “sheveled.” It’s pronounced /di-SHEH-vuhld/, not as you sometimes hear it, /dis-HEH-vuhld/.
(From late Latin in- ‘not’ + scrūtārī ‘to search or examine thoroughly’ + -able. Scrūtārī comes from scrūta)
Inscrutable refers to "something that cannot be searched into or found out by searching; unfathomable, entirely mysterious." But you’ll search harder to find the word scrutable; it’s used mostly in opposition to inscrutable.
(Via French from Latin in- ‘not’ + effāri ‘to utter’)
Ineffable—something "that cannot be expressed or described in language"—can breathe a lonely wordless sigh. Its partner doesn’t come around much any more. Effable once meant "sounds or letters, etc. that can be pronounced." It is used only rarely to mean "that which can be, or may lawfully be, expressed or described in words," or as a snickery double entendre:
She: Are you dumping me? What went wrong? He: I can’t explain. It’s ineffable. She: Are you saying I’m not f—able?
Disappoint was once was the negative of appoint. It meant "to undo the appointment of; to deprive of an appointment, office, or possession; to dispossess, deprive." It was used that way in 1489, but by 1513, it was stretched to its present meaning: "to frustrate the expectation or desire of (a person)." You wouldn’t know the two words were once partners.
You know about indelible ink and indelible memories, but when have you heard of anything being “delible”? During the 17th and 18th centuries the word delible, meaning "capable of being rubbed out or effaced" was used, but it’s gone without a trace. It was delible.
(From late Latin impeccābilis, from im- ‘not’ + peccāre, ‘to sin.’)
Although impeccable now means "adhering to the highest standards" and we speak of impeccable manners or taste, originally it meant "not capable of or liable to sin." These days, peccable is used only facetiously, as in this 1992 quote from the New York Times: “Its credentials are about as impeccable as you can find in the peccable atmosphere of Hollywood.”
(From late Latin indolent, from in- ‘not’ + dolere, ‘suffer or give pain.’)
When it entered English in the 17th century, indolent meant "causing no pain." Doctors spoke of an indolent tumor or ulcer. Maybe some folks misinterpreted the meaning as "inactive," but somehow in the 18th century, indolent gained its current meaning in reference to people: "lazy or idle." The word dolent, meaning "sorrowful or grieving," existed for a few centuries, but it’s obsolete now and never meant the opposite of present-day indolent.
(Via French, from Latin in- ‘not’ + dēfatīgāre ‘to wear out’ + –ble ‘able to’)
An indefatigable person is "untiring; incapable of being wearied." The word defatigable, "capable of being wearied," exists, but it’s too beat to show up very much, leaving indefatigable pretty lonely.
(Via Old French, from late Latin in- 'not’ + cessant- ‘ceasing’)
Incessant refers to something unpleasant that continues without pause or interruption. Cessant was around briefly in the 17th and 18th centuries, but it has ceased to appear these days.
(From the Old English reccelēas, from the Germanic base reck, an archaic word meaning ‘care.’)
Reckless describes a person or the actions of a person who acts without thinking or caring about the consequences. There never was a word like reckful to serve as a positive counterpart to reckless, but reckless people have their fill of wrecks.
Disgruntled is a ringer. This time the prefix “dis-“ is not a negative, but an intensifier. If you’re disgruntled you’re extremely gruntled. And what, pray tell, does it mean to be gruntled? “Gruntle” was a diminutive of “grunt,” dating from around 1400, meaning "to utter a little or low grunt." Later it came to mean "to grumble or complain."
Sources: OED [Oxford English Dictionary] Online, New Oxford American Dictionary (Second Ed.), The American Heritage Dictionary of the English Language (Fifth ed.)