The story
Invented by Walter Scott for Ivanhoe, possibly misread for Cerdic, name of a Saxon king, anglicized from Welsh caredig (“beloved”), in which case a doublet of Caradoc, Caradog, Caratacus, Craddock, Cradock, McCarthy, and McCarty. Alternatively Scott based Cedric on the Welsh name Cedrych (“spectacular bounty”).
Etymology from Wiktionary, CC BY-SA 4.0.