Behind fiercely you can picture an animal that will not back off: fur up, eyes fixed on you, a low sound in its throat. Nothing has been hit; the whole thing is heat and refusal to give way. English sends that heat in two directions. It can stay on the face and in the voice — a hard stare, a sharp word. Or it can move into how hard something is done or how far a quality goes: opposed hard, competing hard, independent to the last inch.
From the word’s history
Wiktionary gives fiercely as Middle English fersly, that is fierce + -ly — in a fierce manner. The word has always been about doing something with that heat in it.
Wiktionary 記錄 fiercely 來自中古英文 fersly，等於 fierce（兇猛的）加上 -ly（…地），也就是「用兇猛的樣子做某件事」。這條線一直沒斷，今天的兩個用法都還帶著那股熱度。
Wiktionary entry 'fiercely' (adv), etymology_text in packet: 'From Middle English fersly; equivalent to fierce + -ly.' (kaikki:3f6f69a190036ae4:1025690247)
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