fat/slim chance

[sitifan]

Why does fat chance and slim chance mean the same thing?

Her chance of getting well is very ______. (A) heavy (B) slim (C) fat (D) thin
The answer to the above question is option B. Is option C also acceptable?

[Barb_D]

No.

You can say "fat chance" to mean "slim chance" because you are speaking ironically, but you would not say "her chances are fat."

You also wouldn't the sarcastic "fat chance" over a matter of life and death.