The large dog growled fearsomely, making the child immediately hide behind his mother.

fearsomely 是形容词 fearsome 的副词形式，由名词 fear（恐惧）加上后缀 -some（具有...性质的）构成形容词，再加上 -ly 变成副词。fearsomely 意思是"令人恐惧地、可怕地"，形容狗咆哮的方式很吓人。