Take a break or rest from, stop, especially quit working. For example, He knocked off work at noon, or Let's knock off at five o'clock. [Colloquial; mid-1600s] Also see knock it off.
Also, knock out. Dispose of or produce easily or hastily, finish, as in A writer of detective novels, he knocks off a book a year, or We can knock out a rough drawing in a few minutes. The first colloquial usage dates from the early 1800s, the variant from the mid-1800s.
Get rid of, reduce, as in She knocked off twelve pounds in a month, or They knocked off one-third of the original price. [Colloquial; early 1800s]
Kill, murder, as in They decided to knock off the old lady. [Slang; early 1900s] Also see knock someone's block off.
Copy or imitate, especially without permission, as in They are knocking off designer Swiss watches and selling them for a few dollars. [Colloquial; late 1800s]
Hold up, rob, as in The gang knocked off two liquor stores in half an hour. [Slang; early 1900s] Also see knock the socks off.