The Collaborative International Dictionary of English v.0.48:

Flatten \Flat"ten\ (fl[a^]t"t'n), v. t. [imp. & p. p.
   Flattened; p. pr. & vb. n. Flattening.] [From Flat, a.]
   1. To reduce to an even surface or one approaching evenness;
      to make flat; to level; to make plane.
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   2. To throw down; to bring to the ground; to prostrate;
      hence, to depress; to deject; to dispirit.
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   3. To make vapid or insipid; to render stale.
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   4. (Mus.) To lower the pitch of; to cause to sound less
      sharp; to let fall from the pitch.
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   To flatten a sail (Naut.), to set it more nearly
      fore-and-aft of the vessel.

   Flattening oven, in glass making, a heated chamber in which
      split glass cylinders are flattened for window glass.
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