Literally 'flying-fur legs' — legs so fast they sprout feathers. The 'feathered foot' image is borrowed from old wuxia novels where supernaturally fast runners are described as having
飞毛 'flying fur'. Modern bonus meaning:
飞毛腿导弹 (Scud missile) — a phonetic-semantic loan, where 'Scud' sounds vaguely Chinese-fitting and the 'fast missile' meaning is reinforced.