[rewboss]

"Underdog" does have another meaning: it's the participant in a game or sport who, at the start, has the smaller chance of winning, or someone who is a victim of injustice, especially in a social context (e.g. a schoolchild who is often bullied). This meaning makes sense; it's based on observations of the hierarchy in a pack of dogs.

Given the similarity in sound between "underduck" (which sounds like a relatively new invention) and "underdog" in certain dialects, I think it's possible that "underdog" meaning to duck under a swing may be a result of mishearing "underduck", confusing it with an older, more established word. Words often develop and spread before their exact spelling is established in print, which is why regional variations like this are able to develop.

Alternatively, of course, it could be that, for some unknown reason, "underdog" was the original term; some speakers then assumed it should be "underduck" as that would be more logical and more descriptive of the action it denotes.

As for whether either of these terms is "correct English", all we can really say is they don't appear to be listed in any dictionary -- and that, in turn, simply means that they haven't been recorded in any dictionary yet. It might be a neologism -- a new word -- or perhaps non-standard; but it is a word at least in some parts of America.