The boxer dodged his opponent's heavy punch thanks to his quick reactions.

reactions 是名词 reaction 的复数形式。reaction 由前缀 re-（表示"再次"）和 action（行动）组成，字面意思是"对某事的回应行动"，即"反应"。 这里指拳击手对对手出拳做出的快速应对动作。