Frankly, I have no idea.
The first answer which is convincing beyond argument will be accepted.
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When going to a bakery, a shepherd's pie is when you buy 3 pies and also get a slice of one more to try as a sample.
The tradition goes back to the 16th century. Due to a wool shortage, shepherds in England all became incredibly wealthy. When most people bought pies at a bakery, they could carry at most two pies due to having only two hands. The suddenly well-to-do shepherds could carry a third because they would bring their hired help with them to help carry. Bakeries tried to court these customers by standing outside their shops and offering free samples of their baking. When a shepherd tried a pie he liked, he would stop in the store and buy 3. That three plus the one free slice became known as a "shepherd's pie". Incidentally, this free sample practice is also continued to this day in a store known as "Costco".
Shepard's pi is the number
Which looks like it just goes on and on forever, ever changing, but really it's just the same thing repeating.