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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?
flatten someone or something out
to make someone or something flat. If you fall under the steamroller, it will flatten you out. Flatten out that dough a little more. Please flatten it out.
1. To become completely flat: The graph indicated that sales were strong in July but that they flattened out in August.
2. To make something completely flat: I flattened out the crumpled paper and wrote on it. The iron flattened the wrinkly fabric out.
3. To knock someone to the ground: The boxer flattened out his opponent. The bicyclist flattened the pedestrian out.