It wasn't just anybody who said "shut up", either - would you believe, it was the leader of a story time session for under-fives.
We'd gone along to the story time in a bookshop as it had been highly recommended by a good friend of mine, who has a baby around the same age and has been going for months. It started much as I'd expected. My son played with 10 other babies and toddlers while the storyteller, a middle-aged man, sang along to his guitar and read aloud children's picture books.
When my baby started grizzling in a complaining way (just like all babies do, and not loudly or disruptively), the teacher turned to him and said firmly: "Now that's enough of that!'" But obviously this meant nothing to my son, who carried on crying. That was when the man looked at him and said in mock anger, "Hey, you - shut up!"
It was the first time I've ever heard someone be rude to my son, and I was surprised by how furious, hurt and offended on his behalf I instantly became.
I said to my baby: "Don't listen to that man - you can cry all you want." I then picked him up with all our things and stalked off to the other end of the bookshop, to the obvious bemusement of the teacher.
But was I overreacting? My friend could see why I was offended, but felt I was taking the words the wrong way. She said that the instructor, whom she knows reasonably well, is a genuinely nice person who was just joking and not coming across well. She'll be going back - she loves it there - but although I didn't confront the class leader and eventually returned to let my son hear the end of the stories and songs, I will never take my baby there again.
I know he meant it jokingly, but to me, "shut up'" is a harsh phrase; one I wouldn't use to any baby (or adult, in fact), just like I wouldn't tell a child to "piss off" or use other swear words towards them.
For a start, I think it's rude, bad language, and not the sort of thing I want my son to be exposed to. But apart from anything else, I can't see what on earth the point of telling a baby to shut up was - it's not as if he was old enough to be a) doing anything naughty - after all babies do cry; or b) able to understand.
If my son had been crying for long, or loudly, I would have taken him to one side, but he had only been grizzling slightly for perhaps a minute when the man told him to "shut up". It wasn't as if I'd brought a crying baby to a wedding or a play - this was a baby class, and anyone who goes to these as much as I do knows they are always full of crying babies. And even if my son's crying was a problem, all the teacher had to do was politely suggest I take him to one side. Yet obviously he thought it was perfectly reasonable to say "shut up" to a baby; he certainly made no apology.
In general, other mums whom I've told the story to have been outraged on my behalf. In fact, some have told me if they'd been in my shoes, they'd have screamed "How dare you?" and lamped the teacher. (Looking back, I actually wish I'd said something to him).
But perhaps to others, my reaction was prissy, precious and over-protective. So is there certain language which simply should not be used towards children? And how one should respond?
My experience reminded one friend of a four-year-old girl she knows who has been brought up by parents who use the 'F' word around her. The girl now uses the word freely herself, and yet her mother can't see anything wrong with this.
That's an extreme case, and I can't see many parents thinking it's in any way defensible. "I am always getting shocked by the way people speak to their children and how the children in turn then speak to them," says Kelly, a mum of one. "I was in M&S over the summer and a little boy was trying hats on and his dad told him to take it off as it was 'gay' and made him 'look gay'. I was livid!"
But where exactly do you draw the line with offensiveness? A mummy friend of mine told me of how her NCT group distanced themselves from one member after she jokingly called her newborn girl a "little bitch" when describing how she'd kept her up all night.
I could immediately understand why the mothers found her language about her child shocking and distasteful to the extent they didn't want to be friends with her any more. And yet I can also see that it was just meant as a joke and probably came across much more harshly than it was meant.
Perhaps certain tones can make 'bad language' OK. My Canadian friend Janis, a mum of two and director of Reallykidfriendly.com, says: "Where I'm from, exclaiming 'shut up!!' is just a rather exuberant way of saying 'are you serious?', a little like saying 'get outta here!' or 'no way!'.
"Out of context, and if you can't see the grin starting to spread across my face, it must sound like I am the worst mother in the world. No doubt about it, admitting that you occasionally tell your children to 'shut up' sounds awful on paper. But trust me, it's all in the tone... and if you ever catch me saying it to my kids with a voice filled with anger rather than humour, please just tell me to shut up."
Perhaps circumstances also make a difference, too. What about if your child is not an innocent baby, but a disrespectful teenager, and they are being extremely badly behaved? A parent, or a family friend such as a godmother, might well end up shouting "shut up!" in exasperation in the heat of the moment.
My friend Catherine, a mother of two older children, confesses: "I have to admit to telling my own kids to shut up sometimes (I'm not proud of it). I wouldn't tell someone else's kids to shut up, but if they were in my own house I might ask them to quieten down a bit."
Lisa, a mother of four, feels: "If a very good friend jokingly said 'shut up' to one of mine I'd probably be ok with it. It's all dependent on how it is said and how it is meant."
On the other hand, mum of two Natalie feels as I do. Like me, she was always told not to say "shut up" as a child and she says that today she'd never say it to her boys or anyone else. And Lucy, a mother of two, says: "When a stranger says 'shut up' to a ten-month-old, they are not telling the child to shut up. They know perfectly well that the child doesn't understand them.
They are telling you to shut your child up and they are being quite astonishingly rude and ignorant about ten-month-old children.
"I personally don't like 'shut up' for the reason I don't like most bad language around kids: I don't want my kids saying it back to me."
She adds, though: "I am happy to admit to 'AAAARGH! JUST BE QUIET! BE QUIET! BE QUIET NOW!' though. At my own kids."
Do you say 'shut up'? Where's your line on what's rude and what's meant in a light hearted way?