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Webster 1913 Edition

Noun.

[F.

mésalliance

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.

Noun.

Improper association.

misalliance ‎(plural misalliances)

An unsuitable alliance, especially an unsuitable marriage. [from 18th c.]
- 1932, Duff Cooper, Talleyrand, Folio Society 2010, p. 97:
  A misalliance is more shocking to a Frenchman than to an Englishman, and Talleyrand was very French in his appreciation of the importance of family, and in his insistence upon outward correctness of behaviour.