Short and a bit fat, with a round belly. We use it for a person or an animal whose body looks round and soft rather than tall and thin — a tubby little boy, a tubby cat. It is informal, and it is one of those words that sits right on a line: said about a small child, a pet, or a friend you are close to, it sounds warm, even affectionate. Said about an adult you do not know well, it is rude. Nobody writes 'tubby' in a report or a news story; it belongs to speech and to friendly writing. It is milder and more comic than 'fat', and it always carries the idea of ROUND — a tall heavy person is not tubby. The word can also be a noun, a name you call someone ('Hey, Tubby'), and that use is almost always teasing, sometimes cruel. It is a little old-fashioned now; younger speakers say it less than their grandparents did.