Economise, expletivise!

When you are trying to drive in a nail with a heavy hammer, and hit your finger instead, the short, sharp monosyllable which you employ - and which I strongly suspect that even the Archbishop of Canterbury would employ in the like painful circumstance - comes of an instinctive desire to relieve the over-burdened soul.
Walter Murdoch, Bad Language

All very true of course, but you have to wonder why anyone would need an excuse. For instance, when I want to swear, I get rid of the nail and just hit my finger with the hammer instead. One less thing needed.

UPDATE! - Anyway, if it's so easy to hit your finger with a hammer when you're aiming for a nail, you'd think the best way to hit a nail would be to aim the hammer at your finger instead. But who knows? Kids, try this at home! The only way to find out is through an experiment!