"Be fat city" is a colloquialism that typically means to be in a prosperous or successful situation. Synonyms for this phrase include "be in the money," "be in the black," "be a booming success," "be on top of the world," "be prospering," and "be thriving." These synonyms all describe a situation where someone is doing well financially or achieving significant success. It's important to note that "be fat city" is a rather outdated phrase and not commonly used by younger generations. Nonetheless, these synonyms provide a more modern and diverse way to express the same idea.