Brother-Slayer:

It is a kenning. This represents a person who has killed one of his own kinsmen. This is considered the worst act a person can commit in a kin-based society since relationships among kinsmen were the most defining characteristic of Anglo-Saxon tribes. The death of a kinsman had to be avenged. To kill a kinsman was the greatest crime in the Anglo-Saxon society. Cain, a brother-slayer in the Judeo-Christian Bible, symbolizes the worst kind of person. Unferth, who supposedly is the bravest among the Danish people, refuses to fight with Grendel's mother. This is a shameful act, and the author describes him as a coward and a brother-slayer.