1 also flatten out [intransitive and transitive]
to make something flat or flatter, or to become flat or flatter:
Use a rolling pin to flatten the dough.
The land flattened out as we neared the coast.
to destroy a building or town by knocking it down, bombing it etc [= level]:
Hundreds of homes were flattened by the tornado.
to press your body against something:
I flattened myself against the wall.
4 [transitive] informal
to defeat someone completely and easily in a game, argument etc:
We flattened them 6-0.
5 [transitive] informal
to hit someone very hard