Being in fat city is an idiom that means to be in a comfortable or prosperous situation. There are various other synonmys that one can use to convey a similar message. Some of the most commonly used synonyms for being in fat city include "on easy street," "in clover," "in the lap of luxury," "living the high life," "prosperous," "well-off," and "rich." These words and phrases are used to describe a person or group of people who are financially stable and successful in their lives. They are enjoying life and have access to all the comforts that money can buy. In essence, being in fat city is a way of describing a state of being where one has achieved great success and is living a luxurious life.