To cease, as from work; to desist; to stop.
When you knock off, you finish working.

1. When you knock something off, you either accidentally or deliberately use force to make it fall from a place above the ground to the ground below.
2. When you knock something off, you make something quickly and not very carefully.

1. When you say "Knock it off" to people, you want them to stop doing something that is bothering you.
2. When you knock people off, you kill them.