A more thorough explanation:

Definition: To make an unauthorized copy of another's product, usually for sale at a substantially lower price than the original. It can also mean to murder or rob/burglarize.

• The street vendor was selling knock off designer handbags for a fraction of the cost of the real ones.
• The company was sued for knocking off a popular toy design from a competitor.
• The police are investigating a case where a store was knocked off during the night and all the merchandise was stolen.
• The hitman was hired to knock off the rival gang leader.

The examples illustrate how "knock off" can refer to both illegal copying of products and illegal activities such as murder and theft. In the first two examples, "knock off" refers to the unauthorized copying of products. In the last two examples, "knock off" refers to illegal activities such as murder and theft.