The word "牛鬼蛇神" is formed by combining four characters: 牛 (ox), 鬼 (ghost), 蛇 (snake), and 神 (deity), which create two compound nouns: 牛鬼, meaning an ox-headed ghost, and 蛇神, meaning a snake-bodied deity. These pairs represent mythical, monstrous beings from Buddhist lore, and their combination into a single four-character idiom serves to collectively depict evil or sinister forces through the juxtaposition of these hybrid creatures.