GASSES UP is a term used to describe the process of refilling a vehicle with fuel. The crossword solutions for this term are REFUELS (7 letters) and FUELS (5 letters). Refueling is the process of adding fuel to an engine, such as a car or truck, in order to keep it running. Fuel is a substance that provides energy to power the vehicle. Gassing up is a common practice for drivers who need to keep their vehicles running.