mis·al·li·ance /ˌmɪsəˈlaɪən(t)s/
  不適當的結合,不適當的婚姻
  Mis·al·li·ance n.  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.
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  misalliance
       n : an unsuitable alliance (especially with regard to marriage)