To hit or knock (something), intentionally or accidentally, so that it falls.
As I took the can off the shelf, I knocked down the one beside it.
To demolish.
We knocked down the garden shed when we moved.
At an auction, to declare (something) sold with a blow from the gavel.
The picture was knocked down for £50.
To reduce the price of.
They knocked it down by another £5, so we bought it.
To drink fast.
I love to go down the pub and knock down pints of lager.
(usually passive) To disassemble for shipment.
The furniture is shipped knocked down, so assembly is required.