Ashley's answer is right. Building on that: in order to understand the expression, it helps to break it down: "I've had" + "just about" + "all I can stomach" + "of your slanders" "all I can stomach" means the maximum tolerable amount. In this case, the person is talking about the maximum tolerable amount of slanders, i.e. insults. "just about" means "close to" or "almost" So the person is saying "I have had almost the maximum tolerable amount of your insults." Meaning, just like Ashley said, that they're saying they're right at the limit of what they can tolerate and they are about to get very angry if it continues.

It means that you are sick of it--that you're almost at the point where you can't stand it. It serves as a warning--"I've had just about enough of this" is essentially saying "this had better stop, or I'll be very angry."