A word of advice to anyone who has a frail elderly parent: be sure not to scream the epithet 'axe-murderer' at them in a busy car park on a Saturday afternoon unless you're looking for trouble.

Let's rewind to the previous evening. It's the Christmas party in our city-centre offices and one of my junior colleagues, Nicola, comes over to introduce her boyfriend to me. He's bearded and feral-looking and sounds like he's drunk too much of the free wine.

'Saw you the other day in the street, didn't we, Nic?' he says between mouthfuls of indifferent Shiraz. 'Nic pointed you out to me. Walking along.'

'Oh yeah?' I say because what other response is there?

'Yeah. I said to Nic, didn't I? I thought you looked like an axe-murderer.'

Nicola looks shocked and embarrassed and bows her head and groans. 'Dave! You can't say that!'

'It's OK,' I tell her, trying not to appear ruffled. 'I've heard worse. You should hear what my wife says about me.'

But the remark rankles. Later that night in bed I do in fact mention it to my wife, who laughs it off and tells me I shouldn't be so sensitive.

'I mean I can see what he meant actually,' she continues, rather less than sensitively. 'You do sometimes have a sort of scowl on your face when you're walking in the street.'

'That's not a scowl. That's just my face at rest. That's how I look when I'm not putting on a phoney grin for other people. It doesn't mean I'm scowling inside.'

Perhaps wisely, she lets the matter rest.

The following day, a Saturday, I offer to drive my mum and dad into town to do some Christmas shopping, my dad having recently quit driving on account of his failing eyesight and hearing loss and creeping dementia. Walking from the car park, I find myself repeating last night's wounding comment to them. It's busy and noisy and I have to raise my voice to make myself heard above the tinnitus-like drone of the city streets.

'What's an axe-whatever-it-is?' my dad says as we traipse through puddles of icy slush like the spillage from a broken freezer.

'An axe-murderer. You know, someone who kills people with an axe,' and I mime the act of chopping his head off with the side of my hand to make my meaning plain.

'Oh. I thought you said an axe-moulderer.'

'Axe-moulderer? What the hell's that?'

'Well, I don't know, someone who moulds axes, I suppose.'

I find myself becoming increasingly vexed at my dad's obtuseness and, I must admit, my voice does take on a slight edge of weariness and impatience. 'Right, well if that job existed, then it would be an axe-moulder, wouldn't it? Not an axe-moulderer. An axe-moulderer would be someone who moulds axe-moulders.'

'I thought you said ex-murderer,' my mum chips in. A woman of few words, my mum's contributions to our three-way conversations are usually short but pithy.

'Oh for God's sake! Ex-murderer! What does that mean? You can't be an ex-murderer, it's conceptually impossible! Once a murderer, always a murderer!'

All this is carried on, I would contend, in a lighthearted good-natured manner, despite the stridency of my tone, and it's only belatedly, as we leave the car park, that I notice a number of people staring uncertainly in our direction. Busybodies, some would call them. Concerned citizens, others might say. Well, cut to the chase, when we return to the car park half an hour later with our cargo of shopping bags, we're intercepted by two police officers looking like a pair of rugby prop forwards kitted out for a riot.

'Afternoon,' one of the officers says, addressing the confused-looking pair of potential abductees that I'm shepherding towards the car. He gives me only a cursory, albeit penetrating examination. 'Everything all right here?'

'Is there a problem, officer?' I say, because that's what people in TV cop shows always say at this point, usually after they've just slaughtered their entire family and the family dog.

'You tell me,' he says. 'We got a call. Someone harassing a couple of pensioners. Karate chops were mentioned. Know anything about that?'

'Well yes, that was me, but I wasn't harassing them. They're my mum and dad, I'm their son. I'm taking them shopping,' I add, gesturing to the half-circle of shopping bags steeping in the slush puddles at my feet as if that settles it.

'All the same,' he says. 'Could I see your driving licence please, sir, if you're the one who's driving?'

Some minutes later the matter is resolved to the officers' satisfaction and our civil liberties grudgingly restored, but it leaves me feeling shaken and somewhat disturbed that my public presentation of myself allows, shall we say, a certain latitude for misunderstanding.

'You'd think they'd have better things to do, wouldn't you?' I shout over my shoulder as I drive my parents home. 'I mean do I look like the kind of person who goes round harassing pensioners?'

I tell myself they don't reply because they haven't heard the question.