The structure of this idiom is highly descriptive. The first two characters, 衣 (yī) and 衫 (shān), establish the subject: clothing. The last two characters, 褴 (lán) and 褛 (lǚ), are synonyms that both mean “tattered.” By combining them, the idiom creates a strong, layered emphasis on the state of utter disrepair, painting a much more powerful picture than a single word could.