That evidently means that he was a descendant of Agag, the Amalekite king who was executed by God’s prophet Samuel. So wicked were the Amalekites that they had made themselves enemies of Jehovah and Israel.
רשעותם של העמלקים הייתה כה רבה עד כדי כך שהם הפכו לאויביהם של יהוה ושל עם ישראל.