I'm trying to figure out how saying "Shut Up" became such a terrible thing to say - to some, almost like using the "N" word around blacks. The horrified expressions I've seen emerging from adults on the receiving end are - well - priceless.
KNOWING that, I'll use it sparingly, and since I don't have kids yet, I can't comment on how and when I'll use it. Most times I exercise restraint, because even though I don't see why it should offend so, it's clear it does.
HOWEVER...
I've been known to say it loud and clear when someone in my earshot is saying something very deserving of a such a reply. If I hear someone ridiculing another person to their face - and the person being insulted is being hurt by it - you're damned right I'll tell them to shut up. If I'm met by someone who thinks it's funny to start telling racist jokes - I'll put up with it for a bit - but eventually I'll tell them to shut their piehole - not because I'm a holier-than-thou azzhole, but because *I'm* being insulted by someone who has mistaken ME for somebody who thinks that's funny.
And, patience notwithstanding, I'll use it on someone who has gone off the deep end with an hysterical rant - about any subject.
I've used it on a co-worker who doesn't seem to grasp the phrases "I'm very busy right now" "That's personal, I don't wish to discuss that" "I don't want to hear about that" and other like phrases. I'm not sure a two by four crack in the face would work either, but I HAVE thought about it - but "Shut Up" *has* worked in the past.