When an English speaker talks about a “fat cat,” that person is referring to someone who is of high social standing, or someone with a reputation for success, with the implication that the individual is also wealthy. The term “fat cat” relates wealth to a large waistline and a “fat” wallet. This term is a somewhat derogatory label.
The term “fat cat,” according to language historians, originated in the early twentieth century, at a time when “robber barons” or “plutocrats” controlled much of the wealth of America as well as other developed countries. Many people of lesser wealth used this term in derision in referring to those who they felt did not deserve their wealth and opulent lifestyles. Over time, the phrase has survived as part of overall social observation of class.
In terms of its etymology, the term “fat cat” has much in common with a few other phrases that use the same animal allegory. For example, when someone says “when the cat’s away, the mice will play” they are setting up a contrast between a cat, which represents the boss, and the mice, which represent entry-level or front line workers. Here, the cat is seen as a predatory animal, and one that controls the behavior of others lower in the food chain.
Another phrase has developed around the same allegory to illustrate the reverse of the idea. If someone says “the mice are always working when the fat cat’s about” this links the idea of the predatory cat to the idea of the wealthy or “fat” boss who makes sure that underlings are hard at work. It also presents the same derogatory use of “fat cat” to imply that the top people in an organization have plenty of wealth.
It’s important to note that the use of the phrase “fat cat” is often superficial, and that not all of the individuals who receive this title actually have a lot of net worth. In modern complex financial societies, it’s entirely possible for someone to control a large organization and not have proportionate assets, especially when financial liability is involved. Many times, when investigators look closely into the pockets of “fat cats,” they find that due to debt, fraud, or other issues, that the individual is actually not wealthy, and may in fact owe much more than he is worth. This shows how use of credit can obscure someone’s true financial situation significantly, and how a term like “fat cat” can be misapplied, even if the person's lifestyle or image fits the stereotype.