FLAT'TEN, verb transitive flat'n. 1. To make flat; to reduce to an equal or even surface; to level. 2. To beat down to the ground; to lay flat. 3. To make vapid or insipid; to render stale. 4. To depress; to deject, as the spirits; to dispirit. 5. In music, to reduce, as sound; to render less acute or sharp. FLAT'TEN, verb intransitive flat'n. 1. To grow or become even on the surface. 2. To become dead, stale, vapid or tasteless. 3. To become dull or spiritless.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI