My eight-year-old son, Joel, comes into my office to ask if there's a worse swearword than fuck. "No," I say.

There's a silence. "You're lying," he says.

"There's none worse than fuck," I say.

Joel narrows his eyes. "I know you're lying," he says. He leaves the room.

On Saturday I take Joel to Chessington World of Adventures. What a crappy theme park! None the less, we have a wonderful day together.

"You're a great dad!" Joel says as we drive home.

"And you're a great son!" I reply with a magical twinkle.

We smile lovingly at each other.

"There is a worse swearword than fuck, isn't there?" says Joel.

"Yes, there is!" I say, still with a magical twinkle.

"What is it?" asks Joel.

"It's c..." I begin. I stop. "Uh," I say.

"Tell me," says Joel. "I swear this is just for me. I'll never use it. I just need to know. I will never use it on anyone. I swear. Just tell me."

I feel clammy and hemmed in. "And you won't tell Mum we had this conversation?" I say.

"I promise," says Joel. "Mum will never know."

There's a silence. "I can't tell you," I say.

"Tell me," says Joel.

"I can't," I say.

"Then why did you almost tell me?" Joel yells.

"Because I wasn't thinking responsibly!" I yell. "I was swept up in the magic of the moment."

"You have to tell me," Joel says. "It's only fair."

"Uh," I say. "I, uh... I..."

I look around the car. For some reason we have an old can of Italian lemonade down on the floor.

"It's limone," I say.

There's a silence.

"Limone?" says Joel.

"That's the worst swearword of all," I say. "Limone. But I'm holding you to your promise that you will never use it. OK? Never."

"Limone?" says Joel. He seems disappointed.

"There's nowhere to go after limone," I say. "Limone is the Everest peak of swearing."

Joel looks out of the window.

"You know," I say, wisely, "sometimes the mystery is better than the knowing, wouldn't you say? Sometimes the journey is better than the destination. Anyway, don't tell Mum."

We reach the house. Joel rushes inside.

"Mum!" he yells. "Dad told me the worst swearword of all! I know what it is! Limone!"

My wife, Elaine, appears at the top of the stairs, an inscrutable expression on her face. I shrug, anxiously.

A month passes. We go for a weekend away. At the hotel, a boy on a tricycle crashes into Joel.

"Limone," mutters Joel under his breath.

Another month passes. Joel has a friend round for a sleepover. At 11pm I hear them talking. They're saying, "Limone" in awed whispers.

"I feel terrible about this," I say to Elaine. "I've tricked my own son. I'm going to tell him that limone isn't a swearword, and is in fact the Italian word for lemon." I pause. "I'm going to tell him the actual worst swearword in the world."

"You are not!" says Elaine.

"I'd rather he was foul-mouthed and accurate than see him like this," I say. "All because of my stupid, stupid slip of the tongue in the car on the way back from Chessington World of Adventures."

"You are not going to tell Joel the worst swearword in the world!" Elaine yells.

And so I don't.

Today, Joel comes into my office. "Hi," he says.

"Hi," I say.

There's a silence.

"Anyway, I'll see you later," says Joel. He goes to leave. Then he turns around.

"Oh," he says. "Cunt."