Berserkers (or berserks) were Norse warriors who are primarily reported in the Old Norse literature to have fought in a nearly uncontrollable, trace-like fury, a characteristic which later gave rise to the English word berserk. Berserkers are attested to in numerous Old Norse sources. Most historians believe that berserkers worked themselves into a rage before battle. The name berserker derives from the Old Norse berserkr (plural berserkir). This expression most likely arose from their reputed habit of wearing a kind of shirt or coat (serkr) made from thepelt of a bear (ber-) during battle. The element ber- was sometimes interpreted as berr-, meaning "bare", which Snorri understood to mean that the warriors went into battle bare-chested, or "without armor" as men of Odin.
Theories about what caused berserker behaviour include ingestion of materials with psychoactive properties, psychological processes, and medical conditions. Some scholars believe certain examples of berserker rage to have been induced voluntarily by the consumption of drugs such as the hallucinogenic mushroom Amanita Muscaria or massive amounts of alcohol. While such practices would fit in with rituals usages, other explanations for the berserker's madness have been put forward, including self-induced hysteria, epilepsy, mental illness or genetics.
To "go berserk" was to “hamask” which translates as “change form," in this case, a with the sense "enter a state of wild fury" and one who could transform as a berserker was typically thought of as “hamrammr” or “shapestrong. For example, the band of men that go with Skallagrim in Egil’s Saga to see King Harald about his brother Thorolf’s murder are described as, "all together there were twelve of them in the party, all the hardest of men, with a touch of the uncanny about a number of them...they [were] built and shaped more like trolls than human beings," which is generally interpreted as this band of men being hamrammr.
I'll ask of the berserks, you tasters of blood,
Those intrepid heroes, how are they treated,
Those who wade out into battle?
Wolf-skinned they are called. In battle
They bear bloody shields.
Red with blood are their spears when they come to fight.
They form a closed group.
The prince in his wisdom puts trust in such men
Who hack through enemy shields.