Brad Leithauser has a mostly useless New Yorker blog post about allegedly unusable words, half yawn-inducing roundup of the usual suspects (niggardly, awesome, inflammable), half desperate word-count-inflating expansions of random thoughts that floated through his brain (two paragraphs on puissant!). But he starts with an interesting point I’m fairly sure has never occurred to me, so I’ll pass it on here:
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.
Note his coy suggestion that auto-antonym—a term that has its own Wikipedia entry and (per that entry) was originally coined by Joseph T. Shipley in 1960—might be his own personal word (“what I think of as”), and his use of the almost eighty-year-old Webster’s Second, which suggests that he might be one of those idiots who bears a grudge against the great Third (see this LH post and this Sentence first post). But maybe he’s just cheap.