(also + out) to make or become (something) flat or flatter

Flatten the dough

to press (oneself) flush against a surface

She flattened herself against the wall

to raze (a building) to the ground

to knock (a person) to the ground with a blow

to overcome, subdue, or crush (somebody)

It takes more than a little setback to flatten him

to lower (a note) in pitch, esp by a semitone

to become flat or flatter