Source : Webster's Revised Unabridged Dictionary (1913)

Flat \Flat\, v. i.
   1. To become flat, or flattened; to sink or fall to an even
      surface. --Sir W. Temple.

   2. (Mus.) To fall form the pitch.

   {To flat out}, to fail from a promising beginning; to make a
      bad ending; to disappoint expectations. [Colloq.]