If you've ever been so angry you'd swear you felt your blood boiling, you could reasonably say that you were furious. The word basically means "full of fury," so you get the picture.
Think about the angriest you've ever been. Now double it and you might be getting close to furious, from the Latin furiōsus, meaning "full of rage," which in turn comes from furia, "fury." Greek and Roman mythology had the Furies, who dispensed justice by harshly punishing criminals, sometimes driving them mad. They were merciless, fearsome, and feared, with snakes for hair and bloody eyes. When not on earth, they were tormenting the damned in Hell. Now, that's furious!