'To flatten a sail' definitions:

Definition of 'To flatten a sail'

From: GCIDE

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. [1913 Webster]
2. To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit. [1913 Webster]
3. To make vapid or insipid; to render stale. [1913 Webster]
4. (Mus.) To lower the pitch of; to cause to sound less sharp; to let fall from the pitch. [1913 Webster]
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. [1913 Webster]