How did the prefixes ひ and か come about and what are the differences in meaning between ひ弱い and か弱い?
They both seem to emphasise or strengthen the meaning of 弱い but how are they different in extent or nuance? What are the situations where you would use one but not the other?
(On the grammatical side of things, I noticed ひ弱い, ひ弱な, か弱い but not か弱な. But I'm not sure if か弱な appearing to be forbidden has any significance.)
Also, I came across a か細い. Are the prefixes productive? (Can I also form a ひ細い? Or prefix か or ひ to other adjectives? It would appear not since ひ細い doesn't have a dictionary entry )