Rag has two definitions relevant to desktop publishing, although neither is in wide use.
1) Once a common material used in the papermaking process rag referred to the amount of cotton fiber relative to the other materials in the paper pulp. The term rag content is not used as much today because other materials are used instead for making paper.
2) The uneven alignment of text along the right (ragged right) or left margins is known as rag. Text with a lot of rag has very uneven line lengths. Although this meaning is more prevalent than the paper meaning for rag, ragged is the more common form for this usage.