The phrase "was in fat city" is a colloquialism that means to be in a financially comfortable situation or to be enjoying a high level of success. There are several synonyms for this phrase, including "riding high," "living large," "rolling in dough," and "in the lap of luxury." You can also use "on top of the world," "prosperous," or "flourishing" to describe this state. Other phrases may include "making a killing," "hitting pay dirt," or "striking gold." These expressions all describe a favorable situation or a period of success, and can be used in a variety of contexts, from financial to personal.