Doing something ferociously means acting in a fierce or violent way. When you go next door to meet the neighbor's new puppy, you'll be startled to hear it bark ferociously at you over the fence.
There's a kind of savagery to this adverb, which describes things done in a wildly violent way. Imagine a tiger roaring ferociously or a wolf ferociously baring its teeth. You can also use this word in a figurative way to mean "extremely." You might be ferociously competitive at chess, for example, or describe your best friend as ferociously loyal. The Latin root, ferus, means "wild."