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.
Via Maria Pia Montoro