Flat \Flat\, v. t. [imp. & p. p. Flatted; p. pr. & vb. n. Flatting.]

1. To make flat; to flatten; to level.

2. To render dull, insipid, or spiritless; to depress.

Passions are allayed, appetites are flatted. --Barrow.

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

Source: Webster's Revised Unabridged Dictionary (1913)