Fat chance is sarcastic, slim chance is direct

The phrase "fat chance" was likely originally used sarcastically. A quick search doesn't show any consensus, though.

According to this stackexchange:

fat chance is like a cynical no chance:

You think you can win the lottery? Fat Chance.

slim chance is when there is a small possibility. It does not infer any hidden meaning.

Even though I did not study, there is a slim chance that I will pass the test.

"Are you up for it?" and "Are you down for it?" both mean the same thing too

They don't mean the same thing as I understand them. Slim chance means there's a chance but fat chance means practically no chance.

Fat chance is ironic, slim chance is not. it's just that nobody uses fat chance in a non sarcastic manner.

because the persons who came up with those phrases were Biggie Smalls and FatBoy Slim

Full sayings:

• slim chance
• fat chance in hell

The "in hell" gets dropped from fat chance making the colloquialism confusing.

This is similar to the "blood is thicker than water" shortening of "the blood of the covenant is thicker than the water of the womb", people who have always heard the first one often use it as an argument for " family sticks together "; when the saying really means the bonds we form of our own choosing are stronger than any presumed by family.

Slim chance is the objective way of expressing a low likelihood. Fat chance is a colloquial expression using irony. You say one thing but mean the opposite. It's more often used as just a quippy two-word reply.

Language doesn't make sense. When flammable and inflammable can mean the same thing, all bets are off. You cannot make sense of these things; you can only accept them.

Likewise, “all but impossible” sounds like the probability is anywhere above zero (0, 1]. Counterintuitively, what it really means is that it is actually very close to zero. So maybe something like [0, 0.1]. Should zero be included in this interval?

Fat Chance is pretty much always sarcastic. However most other common sarcastic phrases are also used sincerely, so it's actually kind of unique. You could say Phat Chance though which would probably be sincere.

I think "fat" is sarcastic. At least that's how it reads to me.

They're not the same though. "Fat" chance carries lard/tallow which can burn, so it is flammable. "Slim" chance has no lards or greases to burn, therefore it is "inflammable". /s

Because English is three languages in a trench coat pretending to be one language.

Wait until you find out a grill utensil is called a spatula but a tool to mix baking ingredients is also called a spatula.

Because fat chance is said sarcastically

This is how I felt when I tried to win an argument about 'deceptively deep' and found that usage was subjective and that 'deceptively shallow' could be used interchangeably

Fat and slim are both as far from the middle of the Gaussian;

"fat" ain't "good", brah. you gotta say "t h i c c"~

jokes aside, it's mostly answered by other good comments here.

A fat chance means the odds are so wide it would be impossible. Like, you have so many slim chances at once on different factors for it to remotely work.

A slim chance means the odds are tight but there is an actual chance on a single factor.

One slim chance is better than 40 slim chances focused on a single goal, so fat chance.

When I use slim chance, I actually will try it if it isn't going to work and it wouldn't hurt to try.

Fat chance means you shouldn't bother to try it. The stars would have to perfectly align.

fat chance is said sarcastically

They dont.

Those expressions are stupid anyway. Say high/low probability instead.

Flammable and inflammable mean the same thing.

Literally means both literally and not literally.