Nadhirawr
Fierce is when someone is like a mad dog with a vendetta and a full tank of gasoline. They don't back down and usually end up yelling at people for no good reason.
You're fierce, bro. You just cursed the entire school.
She was fierce. She challenged the principal to a duel. He didn't show up.
He was fierce. He bit a math problem in half and spit out the answer.