When the Angles, Saxons and Jutes invaded England during the fifth century, they wiped out the native Celts both physically and linguistically. Except for place-names, only a half-dozen or so Celtic words entered Old English. By the ninth century, with the Celts all but exterminated, the Anglo-Saxons settled down to enjoy the good English life.
"Se Doefel Daner,"
Word Ways: Vol. 14
, Article 13.
Available at: http://digitalcommons.butler.edu/wordways/vol14/iss1/13