Posted by Scott Marsden on July 28, 2000
I knew there was only a slim chance of finding out why these two phrases mean the same thing!
Where I come from, the 2 are used to mean similar things (ie. not much chance at all), but are used differently.
Fat chance is more of a standalone statement. One person might ask a question, and the response will be
Slim chance is more often used in a sentence, as in "there is a slim chance of that happening."
I think, as has been alluded to earlier, that they are both regional sayings that became part of the greater store of colloquialisms.