What is the origin of the word "berserk"?

Dear Word Detective: Recently I've been reading about the Vikings, who used to tour Western Europe about 11 centuries ago and who generally didn't pay their hotel bills. In the course of my reading I've come across the word "berserkers" on more than one occasion where it seems to refer to a special type of Viking warrior. I presume this is where the modern word "berserk" comes from. Is this so and how did this bunch of Vikings get their curious name? -- Brian Harrington, Ireland.

Well, you'd go berserk too if you had to eat what those Vikings ate. I recently heard a shortwave program on Radio Deutsche Welle (from Cologne, Germany) about a restaurant in Iceland that offers tourists the opportunity to sample Genuine Ancient Viking Cuisine. I'm actually rather sorry I listened. It seems that one of the Vikings' favorite dishes was something that sounded (this was radio, remember) like "rottenshark." And, by golly, that's exactly what it was. The Vikings used to catch a shark, bury it in the ground for two months, and then dig it up and eat it. And nowhere in that process did they cook it. People apparently still eat this stuff in Iceland, by the way.

"Berserk," meaning extremely and irrationally violent, does indeed come from "berserker," the term given fierce Norse warriors (though I'm not certain that the berserkers were, technically, Vikings). The berserkers were famous for their frenzied ferocity on the battlefield, and this was in an age when even the tamest army was pretty savage by our standards. The name "berserker" comes from the Icelandic equivalent "berserkr," meaning "savage Norse warrior," which is generally thought to be a combination of "ber" (bear) and "serkr" (shirt or coat). A "berserker," according to this theory, would have been distinguished by the coat or shirt of bear fur that he wore into battle, certainly an appropriate wardrobe choice for someone whose intent was to terrify the opposition.

Today we use the term "berserk," usually in the phrase "to go berserk," to describe someone so violently angry that they are completely out of control and beyond reason, or, in the modern vernacular, "going postal."

From http://www.word-detective.com/100699.html#berserk

Another possible, although disputed origin for the word comes from my native Icelandic: ´´Ber´´ can also mean ´´bare´´ or ´´without´´, and ´´serk´´ is ´´shirt´´. The berserker is then someone who is without a shirt. i.e. naked.
This is entirely plausible, there are numerous accounts of berserk warriors in the northren countries charging naked into battle. Romans were often quite unnerved by this sight, when battling the anicent Celts, probably due to the size of their...uh...weapons.
Another fact that makes this theory plausible, is the side effect of the Amanita Muscaria, the Toadstool or Berserkers mushroom that they consumed to achive their aggrivated state. Along with the dementia and hallucinations, it causes profuse sweating, all the more reason to discard clothing.

Concerning Andersens777 answer from last year, I must leap to the defence of my ancestor´s practice of eating rotten shark. It´s not as violently disgusting as it sounds.
Sharks are, or at least were, plentiful in northren waters. They are relatively easy to catch, and a 4 tonne Basking Shark can yield hundreds of kilos of meat, enough to keep a viking family fed through the harsh winter months.
Sharkmeat is however toxic for some part of the year, containing amongst other things ureal liquids and trimethalaminoxids. One of the ways to neutralize these toxins is as follows:
The meat is cut into strips, buried or placed into wooden boxes, and allowed to stand for 3-12 weeks, depending on the type of shark. This is called curing, as the meat does not decompose to any extent, but the toxic elements break down and transform, making the fish safe for consumption. Afterward the meat is hung to dry, an in this state it can be preserved for months, even years.
Sharkmeat prepared this way does have a very pungent ammonia-laced odor, but it has a firm texture, and a surprisingly pleasant, salty taste. It´s Icelandic sushi, basically.
The meat is highly nutritious, high protein, low fat, no carb, with lots of omega-3 fatty acids to make you smart, and keep your arteries clean. It isn´t consumed in large amounts anymore, but used as an appetizer during special Icelandic holidays ( see: tradisjoner.no/text5.html)
If sharkmeat were still popular today I would imagine that athletes and bodybuilders might find it beneficial. Come to think of it, it´s the perfect food for someone who wants to look good standing nekkid on the battlefield.