If we may judge from name and deed, their ferocity was unbounded. Erik Blood-Ax, Harold Bluetooth, and Thorkill the Skull-Splitter were some of their names; and they were worthy of them. They made a business of war, and on select occasions worked themselves into frenzies of rage, howled like wild wolves, and gnashed their teeth on their iron swords. They took the greatest delight in these psychopathic fits.
(from A History of England and the British Empire. What the book has described is the word "beserk.")