Source : Webster's Revised Unabridged Dictionary (1913)

Monsieur \Mon*sieur"\, n.; pl. {Messieurs}. [F., fr. mon my +
   Sieur, abbrev. of seigneur lord. See {Monseigneur}.]
   1. The common title of civility in France in speaking to, or
      of, a man; Mr. or Sir. [Represented by the abbreviation M.
      or Mons. in the singular, and by {MM.} or {Messrs.} in the
      plural.]

   2. The oldest brother of the king of France.

   3. A Frenchman. [Contemptuous] --Shak.

Source : WordNet®

Monsieur
     n : used as a French courtesy title; equivalent to English `Mr'