A Ronald McFatass is anyone who happens to be both morbidly obese and a redhead. They tend to spend most of their time indoors as even the slightest amount of sunlight can either give them instant skin cancer or cause them to combust.
Person 1: Wow, look at that fatty.
Person 2: Yeah they're huge. Huge, and a red head.
Person 1: No kidding. What a Ronald McFatass.