FLATTEN

To reduce to an even surface or one approaching evenness; to make flat; to level; to make plane.

To throw down; to bring to the ground; to prostrate; hence, to depress; to deject; to dispirit.

To make vapid or insipid; to render stale.

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.

To become or grow flat, even, depressed dull, vapid, spiritless, or depressed below pitch.