Definition of "underdog"

underdog

noun
plural underdogs
1. A competitor thought unlikely to win.
  Quotations
  In Athens, the Americans are underdogs to the Chinese and the Canadians (the Canadians!)
  2004 August 30, The New Yorker, page 40
  The visit of a Championship side would not normally send a shiver down their spine but they knew that Wigan were underdogs who would snap at their heels and that they possessed a potent bite if they were not kept on a firm leash.
  2014 March 9, Jacob Steinberg, “Wigan shock Manchester City in FA Cup again to reach semi-finals”, in The Guardian
2. Somebody at a disadvantage.
  Quotations
  Laszlo: Isn't it strange that you always happened to be fighting on the side of the underdog?Rick: Yes. I found that a very expensive hobby, too. But then I never was much of a businessman.
  1942, Julius J. Epstein, Philip G. Epstein, Howard Koch, Casablanca
3. A high swing wherein the person pushing the swing runs beneath the swing while the person being pushed is at the forward limit of the arc.