To cause to fall by striking or colliding with. The strong wind knocked down several trees during the storm.
To reduce the price or amount of something. The seller agreed to knock down the price by 20%.
To demolish or destroy a structure. They plan to knock down the old building to make way for a new shopping center.
To defeat or overcome someone in a competition or conflict. The underdog team managed to knock down the reigning champions in the final match.