What is "bagsecg"

Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king. In 870 or 871 he led The Great Summer Army to England. His forces joined those of the Great Danish Army which had already overrun much of England. He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex. He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great.