To go “berserk” is to go crazy, or mataglap, on the battlefield or elsewhere. Literally, what “berserk” seems to refer to (the etymology is not quite certain) is a particular kind of warlike costume from the Middle Ages, a “ber-sark” or “bear-shirt.” Presumably a bear-shirt incorporated the pelt of the beast (with its head, mouth gaping open to show sharp teeth, as a helmet?), and when you donned it you simultaneously showed off your prowess as a hunter, terrified your enemies, and, entering the realm of totemic magic, put on some of the fierceness of the animal itself. You became a bear.  Possibly, the overall effect was like that shown in the 2015 film The Revenant:

The two elements of “berserk” are interesting. “Serk” or “sark” is a northern English or Scottish version of “shirt.” “Cutty Sark,” meaning “short shirt,” and nowadays best known as a brand of Scotch whisky, was the name of a famous nineteenth-century clipper ship, and before that it alluded to the exiguous garment of a sexy young witch, Nannie Dee, in Robert Burns’s poem Tam O’Shanter:

and if you’re wondering about that strange thing she’s holding in her hand, here’s the explanation.

The other element of “berserk,” “bear,” is simply the Germanic name for the animal. What’s interesting is that in the various European languages the animal has so many different names. In Latin, ursus; in Greek, arktos; both words turn up in the scientific or Linnaean name for the European brown bear, Ursus arctus. In Russian, the name is medved, or “honey-eater”; in English, besides “bear,” we say “bruin” (“the brown one”). Linguists think that all these names are the result of a taboo. That is, the animal was so fearsome that it was dangerous even to speak its name. Better to use some sort of paraphrase like “the honey-eater” or “the brown one,” or an honorific title like “our lord” (in Lappish, the language of the Sami people in far northern Scandinavia) or “good father” (in Yakuts, a language of northern Siberia). “Good father,” you might say supplicatingly to the animal before you drove your spear point in, so that you could take its hide and fashion a bear-shirt, so that at the right place and time you could go berserk.