瑟瑟 is an onomatopoeic reduplicate - it captures both the sound of leaves rustling and the visual of a body trembling. Two lock-and-key collocations:
秋风瑟瑟 (the rustle of autumn wind, a poetic cliché) and
瑟瑟发抖 (to tremble - from cold, fear, or anger). The character
瑟 by itself is an ancient stringed instrument, and the rustling sense comes from the soft sound of its strings.
