The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

   2. The oldest brother of the king of France.
      [1913 Webster]

   3. A Frenchman. [Contemptuous] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Monsieur
    n 1: used as a French courtesy title; equivalent to English `Mr'