pocket protector
/ˈpɒkɪt prəˌtɛktə/Chiefly US
noun
Originally: †a device attached to the opening of a pocket and designed to prevent objects falling out (obsolete ). In later use: a plastic case which fits into a shirt pocket and designed to protect the garment from marking by pens, pencils, etc.
Origin
Mid 19th century; earliest use found in The Times. From pocket + protector.