This is in reference to the biblical story of Abraham, unable to have a child with his wife Sara, instead has a child with Hagar. The child born of Abraham and Hagar's divinely ordained union was Ismael, who with his mother Hagar was later sent into a divinely protected exile upon the successful birth of Isaac by Sara. Christian writers of the Middle Ages believed that Muslims were descendants of a lost tribe that originated with Hagar and Ismael's exile.
In the early Middle Ages, the term Saracen was used by some writers to refer to groups other than Muslims. For example,
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(c) Andrew Holt, October 2005. Permission is granted for electronic copying or distribution in print for educational purposes. No permission is granted for commercial use.