1) Knock(v) means to hit something to make a noise. The most common usage is to knock on a door. (Ex. Don’t knock too loudly.)

2) Knock(n) can also be the sound that is made by knocking. (Ex. I heard a knock at the door.)

3) Knock-off(n) is an idiomatic expression that refers to a low-priced (generally illegal) imitation of a name-brand product (usually clothing). (Ex1. Is that really a Prada bag or is it just a knock-off?)(Ex. 2 The police closed the shop for selling knock-off clothing and accessories.)