The Collaborative International Dictionary of English v.0.48:

Jingo \Jin"go\, n.; pl. Jingoes. [Said to be a corruption of
   St. Gingoulph.]
   [1913 Webster]
   1. A word used as a jocular oath. "By the living jingo."
      --Goldsmith.
      [1913 Webster]

   2. A statesman who pursues, or who favors, aggressive,
      domineering policy in foreign affairs; a bellicose
      superpatriot or chavinist. [Cant, Eng.]
      [1913 Webster +PJC]

   Note: This sense arose from a doggerel song which was popular
         during the Turco-Russian war of 1877 and 1878. The
         first two lines were as follows:
         [1913 Webster]

               We don't want to fight, but by Jingo if we do,
               We 've got the ships, we 've got the men, we 've
               got the money too.
         [1913 Webster]