Cantref Mawr means "The Great Cantref". Each cantref in medieval Wales was divided into cwmwdau, and a typical cantref would be made up of two or three cwmwd. Cantref Mawr was made up of seven cwmwdau, which made it one of the largest cantrefs in Wales. Cantref Mawr was an upland area, bounded by the rivers Tywi, Teifi and Gwili. The area at this period was covered in dense scrub which caused difficulties for invading armies, so that the Cantref Mawr often provided a secure refuge for the princes of Deheubarth when facing an invasion.
Lloyd, J.E. (1911) The history of Wales from the earliest times to the Edwardian conquest (Longman, Green & Co.)