A person who calls themselves fat, but also wants to be taken seriously. Fat is a harsh word, so chubby is a great substitution.
-also gives a chubby person a sense of being sloppy and classy, simultaneously.
man says to his wife, "honey, I'm all dressed and ready for the opera, I'm busting out of this tux, but damn... I'm a bad ass chubby chubberton!"