Casing comes from case (in the sense 'container, covering') + the noun-forming suffix -ing. Case derives via Old French casse from Latin capsa ('box, receptacle'). The historical trail runs through capsa in Latin. Morphologically, casing is transparent: it draws on case (in the sense 'container, covering') + the noun-forming suffix -ing. Used as a noun, casing now means "a protective outer cover or shell enclosing something", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in related forms such as case, encase, and casement. The result is a borrowed or transmitted form whose modern spelling carries a compact record of its route into English.