According to Nicholas, Cardigan is a compressed form of the ancient Welsh name Ceredigion, so called after Ceredig or Caredig, a legendary king. After the Statutes of Rhuddlan [Edward I days], it became the English designation of this area of Wales.
Well say hello to Thomas Jones of Cardigan...the first to greet the pages of English documents in this county of Wales.
The history of Cardigan can be found in Nicholas, Vol. I, pp.123-189.