Definition from Wiktionary, the free dictionary
Alternative forms 
berserk (plural berserks)
- A crazed Norse warrior who fought in a frenzy.
a crazed Norse warrior who fought in a frenzy
- Injuriously, maniacally, or furiously violent or out of control.
- After he watched his sister stabbed to death, he went berserk and attacked the killer like a beast or a wild animal.
Derived terms 
injuriously, maniacally, or furiously violent or out of control