The idiom 一蹶不振 is formed by combining four characters sequentially: 一 (one), 蹶 (stumble), 不 (not), and 振 (rise). Literally, it depicts "one stumble, not rise", where 一蹶 sets up a single instance of failure, and 不振 directly negates any recovery, creating a cause-and-effect structure that metaphorically signifies an irreversible decline after a setback.