Definition of underdog noun from the Oxford Advanced Learner's Dictionary

 

underdog

 noun
noun
BrE BrE//ˈʌndədɒɡ//
 
; NAmE NAmE//ˈʌndərdɔːɡ//
 
, NAmE//ˈʌndərdɑːɡ//
 
 
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a person, team, country, etc. that is thought to be in a weaker position than others and therefore not likely to be successful, win a competition, etc. Before the game we were definitely the underdogs. In politics, he was a champion of the underdog (= always fought for the rights of weaker people). opposite overdog
See the Oxford Advanced American Dictionary entry: underdog