"Marble Mouth" is a slang term used to signify people who possess a slight speech impediment. It's hard to describe, but it certainly has nothing to do with location or cultural background like one definition ridiculously stated. It's not that they stutter - they don't necessary. But it's obvious that speaking is not their strong suit and they sound as if they have a mouth full of marbles. Marble mouths, more than anything, are highly annoying because you just want them to spit the words out already.