To knock off work i.e. to stop working, dates from the mid-1600s. To knock something off can mean to do something hastily and without precision, and dates from the early 19th century. Knock off can also mean to reduce the price of something and this usage also dates from the early 19th century. It is also British slang to steal, this usage dates from the late 19th century. To knock someone off is also American slang for kill, which dates from the early 1900s. A knock-off, as in a cheap replica of something, is fairly modern usage and dates from the 1960s.