断肢 is a formal term referring to a severed limb (as a noun) or the act of severing a limb (as a verb-object construction). As a verb-object, it behaves like a separable compound — you can insert elements between 断 and 肢, e.g., 断了肢 ('severed the limb'). It's used in medical, surgical, or traumatic contexts, not in everyday conversation.