The term berserker comes from Old Norseberserkr (plural berserkir), meaning "bare of shirt", meaning a warrior fighting without armor (debated), or "bear shirt", alluding to wearing clothing made of bear fur. Some berserks also took names with björn in them in reference to a bear. The pre-battle ritual of the berserker included ritualistic group chants and dances, serving to work the warriors into a fighting rage; the rituals also seemed to give the berserkers the feeling of invulnerability, causing them to fight with a reckless disregard for their own safety. It is possible that these rituals also included psychoactives (the most commonly suggested psychoactive being the Amanita muscaria species of mushroom) mixed with mead, allowing the berserker to disregard pain and wounds in battle.
Another theory about berserkers is that wearing bear or wolf skins served as a symbol of their proclivity for worshipping the spirit of the bear. Siberian and Sámi peoples venerated the bear (see bear worship), although there is no direct evidence of similar veneration in Viking Age Norse culture.
The notion that Nordic Vikings used the fly agaric mushroom to produce their berserker rages was first suggested by the Swedish professor Samual Ödman in 1784. Ödman based his theory on reports about the use of fly-agaric among Siberian shamans. The notion has become widespread since the 19th century, but no contemporary sources mention this use or anything similar in their description of berserkers. Today, it is generally considered untrue or at best speculation that cannot be proven. A UK television programme in 2004 tested the possible use of fly agaric and alcohol by training a healthy volunteer in the use of Viking weapons, then evaluating his performance under the influence of fly agaric or alcohol compared to no influence. It was shown that use of fly agaric or alcohol severely reduced his fighting ability, and the tentative conclusion drawn was that berserk state was achieved psychologically; otherwise, berserkers would have been too easy to kill. Of course, this does not take into account the mindset that the berserker likely would have attempted to place himself in. According to Saxo Grammaticus they also drank bear or wolf blood.