The idiom "大发雷霆" is formed through the literal combination of its four characters: "大" meaning big or great, "发" meaning to issue or emit, "雷" meaning thunder, and "霆" meaning thunderclap; together, "大发" amplifies the action of emitting, while "雷霆" conveys the violent sound of thunder, creating a metaphorical image of someone erupting in intense anger akin to a sudden thunderstorm.