Lard-ass is a very rude American word for a person who is fat. It is not playful and it is not a doctor's word. It is meant to sting. Lard is the soft white fat from a pig, the stuff some people still cook with; ass here is the American insult-ending you also hear in smart-ass. Put them together and you are calling someone a lump of cooking fat — ugly on purpose. You will hear it in angry speech, in crude jokes, and in films that want to sound rough. There is no polite room for it in class, at work, or about a stranger's body. If you need to talk about weight, say fat, overweight, or something kind. This word is only an insult.