From Longman Dictionary of Contemporary Englishunderdogun‧der‧dog /ˈʌndədɒɡ $ ˈʌndərdɒːɡ/ noun [countable] DSa person, team etc that is weaker than the others, is always expected to be unsuccessful, and that is often treated badly Crowds often feel sympathy for the underdog.
Examples from the Corpus
underdog• They like to believe they are underdogs.• And the underdogs fully deserved their win over the Galway men.• Yes, they are the underdog.• As the underdog here, they have nothing to lose and everything to gain.• Did they make fun of him for speaking up for the underdog in school?• But the vice-president is now the underdog in this race.• Pepsi has responded to this multi-pronged assault by playing the underdog card.• Whether his party is a double winner ... or remains the underdog ... is in the hands of the voters. 