"the beaten dog in a fight," 1887, from under + dog. Cf. top dog "dominant person in a situation or hierarchy."

http://www.etymonline.com/index.php?term=underdog


An underdog is a person or group in a competition, frequently in electoral politics, sports, and creative works, who is popularly expected to lose. The party expected to win is called the favorite or top dog. If the underdog wins, the event is known as an upset. These terms are commonly used in sports betting.

In a more broad sense, "underdog" is used in reference to a social or ethnic group which suffers from discrimination, persecution and/or economic disability and which therefore gains the sympathy of public opinion, either nationally or worldwide. Such sympathy has often proved of crucial importance in the struggles of national liberation, civil rights and social justice movements, and such movements sometimes significantly modify their tactics and strategy in order to gain "underdog sympathy".

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Underdog, another name for a breed of dog known as the Turnspit Dog