h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The story behind it is to do with Welsh longbowmen, who were basically mercenaries and fought with the English against the Scots. Splitters!

Anyway, they were fighting the French at some point and the French threatened to cut their index and middle fingers off if caught, as if you've ever seen a bowman in action you'll know these fingers are used to hold the arrow and pull the bowstring back. The English/Welsh won and to annoy the French they all stuck up their index and middle fingers, a cheeky practice that we still use today.

Right then, that's the history lesson done! Now, what's next?

Well, that's about the same story they tell for the middle finger alone.

It's probably been diluted on its way over the Atlantic or something

It's the exchange rate... the $US is falling faster than fingers of a bowman.