The chengyu 一贫如洗 is formed by sequentially combining the characters 一 (meaning "entirely" or "all"), 贫 (meaning "poor"), 如 (meaning "like" or "as"), and 洗 (meaning "washed"), creating a simile structure where 如 directly links 贫 to 洗 to convey the literal meaning of "as poor as if washed clean", thus emphasizing total destitution through metaphorical comparison.