A person, dog, or cat, who is overweight due to overeating or lack of exercise.
Generally, the extra weight will be centered around
the middle (or gut), thus creating a rotund
appearance.
For humans: I can't find a dress shirt that
doesn't look like a tent. It
seems like all these dress shirts are really made for Fatty McFattersteins.
For dogs/cats: My cat lives for
breakfast and dinner and she has an extra pouch hanging down. What a little Fatty McFatterstein.