"What time do you knock off work today?" = finish work. "I'll knock five bucks off the price of the unit." = take five dollars off the price; discount by five dollars. When you deal with phrasal verbs, you need to look at the whole sentence. The context changes the meaning. There is also: "That's not a real Chanel bag. It's just a cheap knock-off." = it's an inferior copy. Even though it's a noun, we understand the action in making a knock-off to be quick, cheap and not thinking about quality.

In this case 'to knock off' means to finish work, usually for the day.