What is the name of John Cougar video where you see a young girl in slip dancing with fat old man white undershirt front trailer?

The song you are referring to is "Jack & Diane" by John Mellencamp (not John Cougar). The video does not feature a young girl dancing with a fat old man. It takes place in a small-town diner and features a young couple, played by John Mellencamp and actress Jennifer Grey, dancing and interacting with other people in the diner.