flatten
To strike someone and knock them down. Can you believe that skinny little kid flattened the bully?
flatten out
1. To become flat. Ah, so the picture did flatten out after being under that big textbook for a few hours.
2. To cause someone or something to become flat. In this usage, a noun or pronoun can be used between "flatten" and "out." I put the picture under that big textbook to try and flatten it out.
3. To strike someone and knock them down. In this usage, a noun or pronoun can be used between "flatten" and "out." Can you believe that skinny little kid flattened out the bully?