The phrase "am in fat city" is an idiom that means to be in a very comfortable or advantageous position. Synonyms for this phrase include "in good shape," "doing well," "prospering," "successful," "flourishing," "thriving," "in the pink," "on top," "in the money," and "living large." These expressions all convey the notion of being well-off, financially secure, or otherwise enjoying the good life. While "fat city" is a somewhat dated slang term, these synonyms are timeless and can be used in a variety of contexts to describe someone who is doing exceptionally well.