The Collaborative International Dictionary of English v.0.48:

Contango \Con*tan"go\ (k[o^]n*t[a^][ng]"g[-o]), n.; pl.
   Contangoes. [Prob. a corruption of contingent.]
   1. (Stock Exchange) The premium or interest paid by the buyer
      to the seller, to be allowed to defer paying for the stock
      purchased until the next fortnightly settlement day.
      [Eng.]
      [1913 Webster]

   2. (Law) The postponement of payment by the buyer of stock on
      the payment of a premium to the seller. See
      Backwardation. --N. Biddle.
      [1913 Webster]