Page "Magnus I of Norway" Paragraph 2

Magnus was the illegitimate son of Olaf II of Norway, later known as Saint Olaf, by his English concubine Alfhild.

According to Snorri Sturluson's Heimskringla, he was named Magnus ( Magnús in Old Norse ) by Sigvatr Þórðarson, his father's Icelandic skald, after Charlemagne, Carolus Magnus in Latin.