The origin of the term "Baker's Dozen"?

Solution 1:

Have you checked Wikipedia?

The oldest known source, but questionable explanation for the expression "baker's dozen" dates to the 13th century [...]. Bakers who were found to have shortchanged customers (some variations say that they would sell hollow bread) could be subject to severe punishment. To guard against the punishment of losing a hand to an axe, a baker would give 13 for the price of 12, to be certain of not being known as a cheat. [...]

Solution 2:

As ‘Brewer’s Dictionary of Phrase and Fable’ explains:

In earlier times when a heavy penalty was inflicted for short weight, bakers used to give a surplus number of loaves, called the in-bread, to avoid all risk of incurring a fine. The 13th was the vantage loaf.