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.
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