Source : Webster's Revised Unabridged Dictionary (1913)

Overween \O`ver*ween"\, v. t. [AS. oferw?nian. See {Over}, and
   {Ween}.]
   To think too highly or arrogantly; to regard one's own
   thinking or conclusions too highly; hence, to egotistic,
   arrogant, or rash, in opinion; to think conceitedly; to
   presume.

         They that overween, And at thy growing virtues fret
         their spleen.                            --Milton.