The term 'BIG RIG ON THE ROAD' can be broken down into two parts: 'BIG RIG' and 'ON THE ROAD'. The first part, 'BIG RIG', is a four-letter word that can be solved with the answer 'SEMI'. A SEMI, or SEMI-trailer truck, is a large commercial vehicle used to transport goods across long distances. The second part, 'ON THE ROAD', refers to the fact that these vehicles are often seen driving ON highways and roads. Therefore, the complete phrase 'BIG RIG ON THE ROAD' can be interpreted as a SEMI-trailer truck travelling ON a ROAD.