'underdog' originates from the 19th-century dogfighting term, where the 'under dog' was the dog on the bottom in a fight.

'Underdog' changed from the literal sense of a dog in a losing position to a metaphorical sense of a person or team with little chance of success.

Initially, it meant a dog in a losing position, but over time it evolved into its current meaning of a person or team with little chance of success.