1. Fat is an adjective that describes a person or thing that is overweight, or having an excessive amount of body fat.2. The adverb form of fat is "fatly," which is used to describe actions or states related to being overweight or having excess body fat.3. The comparative form of fat is "fatter," used to compare the degree of overweight or amount of body fat between two individuals or things.4. The superlative form of fat is "fattest," used to indicate the highest degree of overweight or amount of body fat among a group of individuals or things.5. "Fatness" is the noun form of fat, referring to the state of being overweight or having excess body fat.