What does a Bakers Dozen mean?, Origin and meaning

The meaning is the easy part, a set or group of thirteen.
Firstly, a dozen means a group or set of twelve. The word dozen appears to be from French, derived from the Latin for Duo (two) and Decem (ten).
The origin part of the 13.
Bread has been the basic food source for millions of people for a very long time. To cheat someone in providing bread was a very serious offence. Historically seen as stealing from the poor. Severe penalties were imposed on bakers who cheated ordinary people by supplying less than was promised. Punishment for offenders was harsh, even brutal. During ancient times a baker could have his ear or hands chopped off for this offence. The bottom line, don’t steal from the poor.
During English medieval times, bakers were regulated by a guild, “The Worshipful Company of Bakers.” The law that bakers had to deal with was the ‘Assize of Bread and Ale”. This law made provision for serious punishment for bread that was sold underweight. Not the number of loaves, but the weight. Floggings etc were the order of the day if less was provided than paid for. Still no reference to a 13 loaves, but this is still to come.
To make sure that they were not subject to floggings, bakers would add a little extra as a form of insurance against underweight. This was to ensure that the correct weight was provided, not the number of loaves. This extra portion was known as the “in bread” or “long measure” to avoid a “short measure”
The phrase “Bakers Dozen” appeared in writing a couple of hundred years later and meant 13 loaves instead of 12, in definition of the practice.
Bakers Dozen also known as a Long Dozen, or a Devils Dozen (unlucky thirteen).
If you want to be a wise ass in a conversation you can refer to a Bakers Dozen as 14, and cannot be proved wrong. There are references to 14 loaves as well. The extra bit provided was never defined, and it would appear as though some bakers even provided a bit on top of the extra.
Most probably those with a very low pain threshold.
But, there is also the story that maintains that the 13th loaf was the profit of the trader. He bought 12 and the 13th was his profit margin. Not many references to this though.
A further reference for the 13th loaf is to make up for shortfalls. Loaves stolen, quality control etc etc.
Image from Wikipedia