撒腿 is a separable verb (离合词) meaning 'to start running' or 'to bolt' — it describes the sudden action of taking off at a run, usually to flee or chase. The verb 撒 ('to let go') and the noun 腿 ('leg') can split: you can insert a resultative complement or aspect marker between them, like 撒腿就跑 ('take off running immediately'). It's colloquial and vivid, often used in narratives about escaping or pursuing.