1) Slang for a fake copy 2) Slang for to rob or steal 3) Slang for to stop, to decist 4) Slang for to kill 5) Slang for a police raid 
• (n.) Act or place of knocking off; that which knocks off; • (a.) That knocks off; of or pertaining to knocking off. • (n.) a cam or the like for disconnecting something, as a device in a knitting machine to remove loops from the needles.
A way of mounting parts such as wings, tailplane or undercarriage so that they will knock-off without damage in the event of a crash landing 
An unauthorised copy of a product, usually designer clothing
Knock'-off` noun Act or place of knocking off; that which knocks off; specif. (Machinery) , a cam or the like for disconnecting something, as a device in a knitting machine to remove loops from the needles. 
Knock off is slang for to kill.
verb stop pursuing or acting; `drop a lawsuit`; `knock it off!`
To knock something off is to steal it, not to copy it!