越俎代庖 is formed by directly combining four characters: 越 (to overstep) modifies 俎 (a ritual vessel), and 代 (to replace) modifies 庖 (cook), creating a literal sequence meaning "to overstep the ritual vessel and replace the cook". This structure originates from a specific anecdote in the classical text Zhuangzi, where the characters are used in their literal senses to depict a concrete action, thereby forming a fixed idiom that metaphorically conveys usurping another's role.