The word "胖子" is formed by combining the root character "胖", which carries the core meaning of "fat" or "plump", with the suffix "子", which is a very common and neutral nominalizer. This suffix "子" attaches to the descriptive root to transform it into a concrete, colloquial noun referring to a person characterized by that trait. The construction creates a familiar, everyday term for "a fat person" or "chubby fellow", where the suffix adds no independent meaning but completes the word, giving it a standard, recognizable form in the spoken language.