What does "flatten" mean?

Additional senses

1. 2.To make vapid or insipid; to render stale.
2. 3.(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.
3. 4.To become or grow flat, even, depressed dull, vapid, spiritless, or depressed below pitch.