I was seeking a replacement for “unfathomable.” I thought of “depthless,” but, feeling a bit iffy about it, I consulted my old Webster’s Second. Yes, it was a synonym for “unfathomable” (“Of measureless depth … unsoundable”) but also for “fathomable” (“Having no depth; shallow”). The word was what I think of as an auto-antonym (a term that doesn’t appear in Webster’s Second): it’s its own opposite. Which is to say, it’s a mostly unusable word.

Suppose in a novel you encounter the phrase “Rick stared into Sheila’s beautiful, depthless eyes.” Rick has clearly met a babe—and she is either superficial or profound. No telling which, outside of context. In its flexibility, its complaisant wish to go both ways, the word loses its independence and leaches away most of its efficacy.

I don’t know how many auto-antonyms English offers, but the list includes “cleave” (unify or sever—the butcher’s wife cleaves to the butcher, who cleaves the cow’s carcass), “overlook” (oversee or fail to notice), “let” (allow or, as in the legal phrase “let or hindrance,” obstruct), “enjoin” (encourage or prohibit), and “sanction,” as in any sanctioned imports are either approved goods or contraband. A lengthy, but not exhaustive, list of auto-antonyms can be found on Wikipedia. (There’s a special appealing subclass of auto-antonyms that exists only when spoken, as in raze/raise a building or—if muddily enunciated—prescribed/proscribed drugs. Something similar arises in Elizabeth Bishop’s poem “Crusoe in England,” with its pun on Mount Despair/Mont d’espoir.)

But most of these tricky little double-talkers manage, through surrounding context or attendant preposition, to fix unambiguously which polar meaning they intend. The beauty of “depthless,” it occurred to me, is its utter reversibility. Unusable? I suppose it might find usefulness through its cover-your-tracks slipperiness: “My daughter’s new boyfriend, Freddy, has a depthless mind.”

Words become unusable for all sorts of reasons. Though “niggard” and “niggardly” have a rich pedigree running through Chaucer and Shakespeare and Browning, they’ve recently fallen out of currency as the result of being near-homonyms to a hateful epithet. On the other hand, a cluster of earthy terms that used to be unusable, at least in civil discourse, has gained acceptability, especially among the young. Not long ago, teaching a course in the novella to undergrads, I was apparently the only one in the classroom who felt there was anything odd or untoward when a shy, soft-spoken sophomore raised her hand to offer this assessment of Edith Wharton’s put-upon and pitiable hero Ethan Frome: “I think Ethan’s a total asshole.” Though the seventies, when I was in college, are recalled as a freewheeling and iconoclastic era, back then “asshole” wouldn’t have been deemed an acceptable lit-crit characterization.

Poor, crippled Ethan—the epitome of patience and interminable suffering—no doubt shoulders this latest indignity with mute forbearance; after all, as his example tells us, life is a process of steadily accumulating burdens. I’m not sure the soignée ghost of Miss Wharton, though, floating about in her monogrammed sheets, regards the trend of our language with the same imperturbability.

Words also can become unusable, paradoxically, through excessive usefulness—overuse. “Awesome” strikes me as an all but unusable word, except in irony, now that we live in a world in which you might plausibly hear an oatmeal cookie or a shoelace described as awesome. (“Awful,” né awe-full, went in an analogous direction but died in a different way.) Likewise, “amazing” and “totally.”

In the three decades I’ve been teaching, I find that the subset of students is steadily increasing who consider nearly all words of three or more syllables pretentious, with the exception of “pretentious”—which is for them an indispensable term. Meanwhile, in recent years, a word I’ve always been fond of—“artisanal”—has been swallowed by an enormous maw. Its fine associations of individual tooling and subtle calibration—the craftsman’s guild—have all but vanished. The word has been devoured by fast-food franchises, supermarkets, junk-food confectioners; any day now I’ll spot on some diner menu artisanal s’mores, artisanal pigs in a blanket.

Occasionally, a word becomes unusable because some writer wholly appropriates it, embedding it into a jeweler’s setting of such brilliance that any subsequent use seems both allusion and dilution. It’s hard to imagine any writer effectively taking up the verb “incarnadine” after Macbeth’s “The multitudinous seas incarnadine.” Shakespeare owns the word. Robert Frost felt much the same way about Keats and “alien.” The depiction of Ruth in “Ode to a Nightingale” (“She stood in tears amid the alien corn”) put such a memorable, unexpected—indeed, alien—spin upon “alien” that succeeding poets risk sounding plagiaristic when employing it.

I suppose an opposite process sometimes unfolds and a word previously unusable because of its tight associations with a particular writer slowly gains a broader, untethered currency. There must have been long decades when to speak of “pandemonium” was inevitably to conjure up Milton, who in “Paradise Lost” coined it for the capital of Hell. But in time, the word floated up from the underworld, entering a serviceable purgatory, and nowadays you hear it constantly on TV, where newscasters are forever identifying pandemonium in the streets, sportscasters finding it on the athletic field.

I suspect that “inflammable” will soon go up in smoke. There appears to be growing confusion as to whether the prefix “in” serves as intensifier (highly flammable) or negator (fire resistant). We’re probably only one big lawsuit away from the word’s near-extinction. Picture the poignant plaintiff, about to receive a multimillion-dollar settlement, explaining in broken English that he bought his daughter a blouse made of an inflammable fabric because he wanted to protect her. Picture the clothing manufacturers racing to alter their labeling.

Long ago, slogging through some now-forgotten academic essay, I encountered “incomplex” and joyously realized I’d stumbled upon another unusable word. Quick, now: Does “incomplex” mean more complex than usual, or does it mean simple? By the time you conclude that it means simple, you’ve also recognized that it’s a laughably unsimple way to say so.

“Incomplex” belongs to my favorite subgroup of unusable words, those that seem inherently, structurally maladapted for the meaning they would convey. Two other examples are “pulchritude” and “puissant.” Pulchritude comes to us directly—incomplexly—from the Latin pulcher, “beauty,” but somewhere in its journey from one language to another it picked up a great freight load of ugliness. Boy, it has a nasty sound. A native speaker of Spanish tells me that the word in his language, pulcritud, isn’t so off-putting. Perhaps it’s the echo of “puke” that renders it so repellent in English. In addition, it reeks of the bombast of outdated theatre. I imagine some long-ago carnival barker: “Gennelmen, step this way, step forward, gennelmen, into this here tent, where two thin dimes bring you a pa-rade of pul-chri-tude not to be sur-passed!”

As for “puissant,” let your burly neighbor overhear you describing his son, the high-school football star, as a “puissant quarterback” and you’re apt to wind up getting socked in the jaw. Surely the word was always meant to signify not the tough and hardy but the weak and wussyish. (The shy “pussy” hiding within it only enhances this impression.) The word came over to English from the French in the fifteenth century. In retrospect, given all our stock images of the fey Frenchman, it was a dimwitted idea to go francophone when seeking to evoke rugged power.