Phat is a slang word that young people, especially in African American English and then in hip-hop, used to mean fashionable, attractive, or really good — a phat beat, phat shoes, a phat rhyme. It is a playful misspelling of fat. In that slang, fat was already being used as praise (a fat contract, a fat beat: thick with goodness), and someone added the h on purpose. It is informal, marked, and you would not put it in a school essay. Though actually, the word has aged in the open. Many younger speakers now use it only as a joke about the 1990s, and outside that music world it can land as a typing mistake. It has nothing to do with body size in this meaning, even though that is the joke. If you need “overweight,” write fat, or a kinder phrase; if you need “excellent,” a living word will be understood more widely now.