It's used as a term of endearment to a grown woman, especially if she tends to feel old. Your "Daddy" used by a man toward a woman can mean either a rich husband or rich boyfriend (sugar daddy), sometimes an older wealthy man.
...but there's no rule that people have to interpret a song as the artist intended it at the time. Many actually prefer that people apply whatever meaning to their songs that makes an impression on them. Works of creative writing can be similar, but we know that already.