Fat is the opposite of slim, so how is it that if there’s a "slim chance" of something happening, it seems just as likely as something with a "fat chance"? Which chance came first, the fat one or the slim? Does the size of the chance have an affect on the probability of the outcome? Well, as you might have guessed, sarcasm played a role in the etymological evolution of these two phrases.
The original unlikely scenario
The phrase "slim chance" is quite a bit older than "fat chance": according to Google's Ngram Viewer, the first instances of "slim chance" being written in books were in the 1740s. "Fat chance" appeared in newspaper reports in the late 19th century, but in the context of there being a large or sizable chance of an event occurring. The more sarcastic read or intonation of the phrase began appearing around 1900.
It’s pretty intuitive to see where "slim chance" comes from. The margin of success is considered small or narrow, i.e. slim. But just because whatever it is seems unlikely, that doesn’t mean that it’s impossible. "Slim chance" does leave some room for possibility.
Imagine that your coworker says, "There’s a slim chance we could meet on Monday, but Wednesday looks much better." A Monday meeting might not look good, but it shouldn’t be ruled out. There’s still a small chance it could happen.
A matter of irony
Enjoying sarcasm as we do, of course people became snarky when implying that something likely wasn't going to happen. Instead of using the more reasonable "slim chance," ironically saying "fat chance" added some extra sardonic bite to their expression. On down the road, the two sayings became more or less interchangeable for a situation having low probability.
Typically, "fat chance" is used only in informal settings. "Slim chance" remains the more appropriate, agreeable phrase, especially in news or written reports.
The two phrases don’t really mean the same thing
While they’re frequently used interchangeably in speech, the two sayings don’t actually share a meaning. It seems like “fat chance” has a somewhat lower probability – almost impossible – compared to “slim chance.”
"Slim chance" leaves some possibility. Chances are small, but it still could happen. Sometimes, "slim chance" is even used optimistically or to encourage. If your sports team is losing the big game, you might reassure your friends, “Don’t give up yet. There’s still a slim chance that they could pull this off!”
"Fat chance" leaves essentially no possibility and is almost always used negatively. If your ex asked you out on a date after a rough breakup, you might reply: "You think we should we get back together? Fat chance of that happening!" That date isn’t going to happen. There’s absolutely no chance, but you wanted to say it with some extra oomph.
When to use which phrase
If you’re writing a professional paper or talking to a work colleague, you'll probably want to stick with “slim chance.” It’s nicer and not as absolute. The phrase “slim chance” is also more commonly found in print due to its more professional tone. It’s also the only proper phrase when referring to a situation that could actually happen. If there’s absolutely zero probability of something happening, “fat chance” is the more accurate term.
If you’re around your friends or in an informal setting, go ahead and use the sarcastic “fat chance.” It’s also great for when you need some extra emphasis on how impossible a situation is. These days, “fat chance” is basically synonymous with “never ever.”