"cc" means "carbon copy"
Carbon copy - Wikipedia, the free encyclopedia
Carbon copying, often abbreviated to c.c., was originally the technique of using carbon paper to produce one or more copies simultaneously with the creation of paper documents. On a typewriter, this would be done by placing carbon paper sheets between two or more sheets of paper in the machine, so that whatever was typed on the front sheet was copied onto the other sheets. This technique applies to written documents as well.