The phrase "a baker's dozen" means 13 instead of the usual 12. This term has an interesting historical origin, rooted in medieval English practices and regulations. 

During medieval times, there were strict laws in England regarding the weight and price of bread. These laws were designed to prevent bakers from cheating customers by selling underweight loaves of bread. One of the most notable laws was the Assize of Bread and Ale, which regulated the weight and quality of bread and ale. 

Bakers who were found to be shortchanging customers by selling underweight bread could face severe penalties, including fines, flogging, or even having their hands cut off. To avoid these harsh punishments, bakers adopted the practice of adding an extra loaf to every dozen they sold. 

By giving 13 loaves instead of 12, bakers ensured that they were not accused of selling short weight. This extra loaf acted as a safeguard against accidental underweight loaves, ensuring compliance with the regulations and avoiding punishment. The practice of including an extra loaf led to the term "baker's dozen." 

The phrase is believed to have been in use since at least the 16th century. It was a practical solution to a serious problem, ensuring that bakers stayed on the right side of the law.