Source : Webster's Revised Unabridged Dictionary (1913)

Misalliance \Mis`al*li"ance\, n. [F. m['e]salliance.]
   A marriage with a person of inferior rank or social station;
   an improper alliance; a mesalliance.

         A Leigh had made a misalliance, and blushed A Howard
         should know it.                          --Mrs.
                                                  Browning.

Source : WordNet®

misalliance
     n : an unsuitable alliance (especially with regard to marriage)