from The New Yorker:
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.