"While there are a few possible stories that link the term to bakers being bad at math or simply being generous, the most widely accepted origin comes from around the (go figure) 13th century. Back then, dishonest bakers scammed customers by overcharging for smaller loaves.

In 1266, King Henry III appointed a legal body known as the Assize of Bread and Ale to regulate the sale of bread by quality, weight, and price relative to the cost of wheat. Any baker caught violating the laws associated with this body would be subject to fines, beatings, or even imprisonment. In order to not be flogged, whenever bakers sold 12 loaves, they included an extra (typically smaller) loaf or end piece of bread as insurance that they hit the weight requirement."