The Collaborative International Dictionary of English v.0.48:

Flat \Flat\, v. t. [imp. & p. p. Flatted; p. pr. & vb. n.
   Flatting.]
   1. To make flat; to flatten; to level.
      [1913 Webster]

   2. To render dull, insipid, or spiritless; to depress.
      [1913 Webster]

            Passions are allayed, appetites are flatted.
                                                  --Barrow.
      [1913 Webster]

   3. To depress in tone, as a musical note; especially, to
      lower in pitch by half a tone.
      [1913 Webster]