The term 'Highway Rig' is a job description for a SEMI-truck driver. A SEMI-truck driver is responsible for transporting goods and materials from one location to another, often over long distances. They must be knowledgeable of the rules and regulations of the road, as well as the safety protocols associated with driving a large vehicle. The term 'SEMI' is a four letter acronym for the job title, which stands for 'SEMI-Truck Driver'.