Source : Webster's Revised Unabridged Dictionary (1913)

Monseigneur \Mon`sei`gneur"\, n.; pl. {Messeigneurs}. [F., fr.
   mon my + seigneur lord, L. senior older. See {Senior}, and
   cf. {Monsieur}.]
   My lord; -- a title in France of a person of high birth or
   rank; as, Monseigneur the Prince, or Monseigneur the
   Archibishop. It was given, specifically, to the dauphin,
   before the Revolution of 1789. (Abbrev. Mgr.)

Source : WordNet®

messeigneurs
     See {monseigneur}