King of the English from 978 to 1013 and again from 1014 until his death. His epithet does not derive from the modern word "unready", but rather from the Old English unræd meaning "poorly advised"; it is a pun on his name, which means "well advised".
[...]
The chief problem of Æthelred's reign was conflict with the Danes. After several decades of relative peace, Danish raids on English territory began again in earnest in the 980s. Following the Battle of Maldon in 991, Æthelred paid tribute, or Danegeld, to the Danish king. In 1002, Æthelred ordered what became known as the St. Brice's Day massacre of Danish settlers. In 1013, King Sweyn Forkbeard of Denmark invaded England, as a result of which Æthelred fled to Normandy in 1013 and was replaced by Sweyn. However, he returned as king for two years after Sweyn's death in 1014. Æthelred's 37-year combined reign was the longest of any Anglo-Saxon king of England, and was only surpassed in the 13th century, by Henry III. Æthelred was briefly succeeded by his son, Edmund Ironside, but he died after a few months and was replaced by Sweyn's son, Cnut. Another of Æthelred's sons, Edward the Confessor, became king in 1042.
"[...] I suppose your ancestors often entertained royalty at Manderley, Mr. de Winter?”
[...] “Not since Ethelred,” he said, “the one who was called Unready. In fact, it was while staying with my family that the name was given him. He was invariably late for dinner.”