How did American slang convert smart or smarting to also mean something painful?

Answer 1

The word "smart" in the sense of sharp/stinging pain actually derives from the very same roots as "smart" in the sense of intelligence; it was not a word created by American slang.

The word smart comes from the Middle English word "smerten", which in turn came from Old English "smeortan" (to be painful) and "smeart" (sharp, severe, stinging). As we can see, the original meaning of the word was actually the meaning of sharp pain we still use today! In fact, the meaning of intelligence probably came from the 12th century, when the idea of "cutting wit" or "cutting words" came about, and people who had cutting/sharp wits were generally clever and intelligent.

We can even go further back in the etymology and see that the Old English "smeortan" came from the West Germanic stem "smert-" (as reconstructed by linguists since it was before recorded language), which meant "to pain" or "to bite", and that came from the Proto-IndoEuropean base "smerd-" (also reconstructed by linguists), which meant "to rub" or "to pound". This shows us that the painful sense of the word was definitely there from the beginning.