In a corner shop 20 minutes ago some guy told me that he was "only joking". This was news to me. The original joke couldn't have been more invisible if it had blended in chameleon-like with the crisps and started whistling innocuously. If you can try to imagine the confusion that might result from a knock-knock joke where you're not actually told who's there, it was a bit like that.

The subsequent reassurance that he was "only joking" felt inadequate. It hovered somewhere between a half-hearted apology and slight contempt for my failure to get the joke – but I didn't come out for a laugh, I came out for some AAA batteries. Call me paranoid, but there was also the distinct possibility that he wasn't joking. Maybe there was a grain of truth in what he was saying. Maybe my trousers were actually on fire. (Or whatever.)

You'll be intimately familiar with the Bible, of course. It describes people who say they're "only joking" as mad men "who casteth firebrands, arrows, and death". Which is a bit much – that's the Bible for you, I guess – but within those two little words, "only joking", lies the essence of humanity's failure to make itself understood. Even more than "Oh God, I thought you meant Tuesday week" or "Darling, you know I do, I don't have to say it" – or, for that matter, "Je ne parle pas français." Two people, failing to communicate with each other effectively, exchanging nervous glances that say one thing alone: "I'm not really sure what's going on right now."

I was once in a short-lived relationship where we both said "I was only joking" so often to each other that it was like we belonged to two parallel ancient civilisations, both struggling with verbal expression and having all kinds of trouble with the idea of using humour as psychological relief. Like the boy who cried "wolf", or Hilaire Belloc's Matilda, who yelled "fire", we were the couple who were "only joking", but never amusingly enough for the other person to get it. We'd explore alternatives on the theme, such as "Joke!" or "I was obviously joking" and "You shouldn't take me so seriously", until one of us suggested we split up, which we both instantly knew wasn't a joke and thankfully provided us with the psychological relief we'd been looking for.