People who bet on the underdog were seen as people who were expecting to see the dog do more than most people think they could, and thus were “rooting for the underdog” to win. Soon, this slang went on to define other situations outside of dog fighting.

What does it mean if you root for the underdog?

By rooting for the underdog, the excitement during the game increases. ... People like when a game is close and the players have fought hard for the win, and so people root for the underdog to make a comeback and fight until the end.”

What does it mean to stand up for the underdog?

The underdog in a competition or situation is the person who seems least likely to succeed or win. Most of the crowd were cheering for the underdog to win just this one time.