I think 'copy off' is just a phrasal verb used in English. It basically means the same as 'make a copy'. A similar phrasal verb for this is 'run off'.
For example: "Can you run off a few copies of this document, please?" or "Can you copy off a few of these for me please?".
Jura's example, 'to copy off someone', is another example of a way to use 'copy off', but in a different context.