I think the phrase “hard of hearing” is rather fascinating. It doesn’t make any intuitive sense. While we can think of it in the sense of something having hardened and become less flexible, or in the sense of something having become hard to do, we don’t use this phrase in any other way.
We don’t call someone who is slow hard of learning. If someone is a jerk who spends too much time on 4chan we don’t say they’re hard of caring. 40 year-old men don’t tell their doctors they’re hard of erections. So where does it come from?
Interestingly, if not a little unsurprisingly, it comes from the Middle Ages, circa 1200, where ‘hard of’ means something is difficult to do. Modern English got rid of this construction, but this very specific phrase, ‘hard of hearing,’ has hung on for 800 years. I guess just the kind of phrase that’s hard of forgetting.
