One day, en passant went to the market with 2 chickens to trade for a small cart.  A merchant laughed at him and said, "Those 2 scrawny birds aren't fit to feed a cat."  The passant was enraged by such a grievous insult.  He drew his sword and sidestepping, he sliced off the merchant's head.

The Queen happened to be passing nearby in her carriage and witnessed the astute play by the passant.  She said to her knight, "Go buy those chickens from that poor man and give him this small purse of gold."

Back at the castle, the King was consulting with his bishops when the dinner bell rang.   After saying grace, the King proceeded to fill his plate.  His knight, who was that evening's food taster, took a bite and said, "Your Highness, this chicken is the best I've ever tasted."  The king dug in and was delighted!  He asked the Queen, "How did we come by such a wonderful thing?"

Then, the Queen explained what she observed in the market and how the brave passant defended himself from the insulting merchant.  The King was so impressed, he went to the rook tower after dinner and ordered his pawn to blow the trumpet.  The pawn called out, "Here Ye, Here Ye!  The King is about to speak!"

The towns people gathered round and the King spoke in a commanding voice, "In honor of a poor man who provided us with a feast of cog en vin, who nimbly sidestepped a rude merchant and adeptly defended his honor by slicing off that insufferable peddlar's head, I hereby proclaim a new move in the royal game of chess:  EN PASSANT."

And everyone lived happily ever after.