He waved his arms maniacally, as if he had discovered some earth-shattering secret.

maniacally 是由名词 maniac（疯子、狂人）加上形容词后缀 -al 变成形容词 maniacal（疯狂的），再加上副词后缀 -ly 构成的副词形式，意思是"疯狂地、狂热地"。