Shut up screams urgency. Be quiet says, tone it down or in some cases...whisper. Shut up means stop talking right this instant, close your mouth, I don't want to hear another word.
I live in a shut up household.
Recently I've discovered that many people find the words shut up to be offensive and insulting. Shut up is down right rude. People prefer that be quiet or shushing should be used instead. I think it's all in the tone.
"Amaya...shuuuuut...up," Rio says, drawing the sentence out like he's not going to be gullible and fall for any of her tricks again. She can't fool him.
"SHUT UP!" Amaya yells, standing two inches from Rio's face, ears steaming, at her breaking point. Little brothers are so very irritating.
So the tone is rude, I think, not the word. I could shush my children abruptly, dismissively, or hastily in the middle of a sentence and be just as thoughtless. But why? When a soft or firm shut up does the trick effortlessly.