This is an interesting conversation that my wife and I had. It's amazing how 16 years can go by and you still come up with new stuff that you hadn't considered before. Here's the gist of it: Which is worse, 'Shut up' or 'Shut your mouth'?
I happen to think that yelling at someone to shut up is more insulting and demeaning. My wife thinks that yelling at someone, "Shut your mouth" is more aggressinve and therefore a worse thing to do. I won't tell our kids to shut up but I say shut your mouth all the time. My wife will tell them to shut up and when she really loses it she'll yell shut your mouth. The funny thing is, because of the way I see the two phrases, when she's really losing it I think she's finally gotten some self control back.
Anyhow, like most things there is no answer. The reason I bring this up is because we as writers have preconceived notions about what things mean. We could be wrong.