The Collaborative International Dictionary of English v.0.48:

Flat \Flat\, v. i.
   1. To become flat, or flattened; to sink or fall to an even
      surface. --Sir W. Temple.
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   2. (Mus.) To fall form the pitch.
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   To flat out, to fail from a promising beginning; to make a
      bad ending; to disappoint expectations. [Colloq.]
      [1913 Webster]