A nickname used by TJ Kippen for Cyrus Goodman on the best Disney Channel Show to ever exist 'Andi Mack'.
TJ uses 'Underdog' since their famous encounter by the Swings.
Or it's also known as an affectionate name for the only person, a not so scary basketball guy like TJ Kippen, can talk to like this.
"Listen Underdog. You can't do a somersault? I can help you with that."