MONOMANIAC is a 10 letter word that starts with M.

1. \Mon`oma"ni*ac\, n.
   A person affected by monomania.
   
   

2. \Mon`oma"ni*ac\, Monomaniacal \Mon`oma"ni*a*cal\, a.
   [Cf. F. monomaniaque.]
   Affected with monomania, or partial derangement of intellect;
   caused by, or resulting from, monomania; as, a monomaniacal
   delusion.