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.
2. To throw down; to bring to the ground; to prostrate;
hence, to depress; to deject; to dispirit.
3. To make vapid or insipid; to render stale.
4. (Mus.) To lower the pitch of; to cause to sound less
sharp; to let fall from the pitch.
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.