He collects stamps fanatically, spending all his savings on them.

fanatically 是副词，由形容词 fanatic（狂热的）加上副词后缀 -ally 构成。fanatic 来自拉丁语 fanaticus，原指"受神灵启示的"，后来演变为"狂热的、极端的"意思。fanatically 表示"狂热地、极度痴迷地"，形容一种近乎偏执的热情状态。