Real Dictionary

knock down
phrasal verb: If someone is knocked down or is knocked over by a vehicle or its driver, they are hit by a car and fall to the ground, and are often injured or killed.
phrasal verb: To knock down a building or part of a building means to demolish it.
phrasal verb: To knock down a price or amount means to decrease it.

knock down in American English
to hit so as to cause to fall
to sell at auction by a blow of the hammer or to a bidder

knock down in British English
to strike to the ground with a blow, as in boxing
adjective: overwhelming; powerful
noun: an introduction