A baker's dozen, devil's dozen, long dozen, or long measure is 13, one more than a standard dozen. The practice of baking 13 items for an intended dozen was insurance against the items being lower than the statutory weight, or of lower than usual quality, which could cause the baker to be fined.
In the late 16th century a baker's dozen referred to a batch made in which the customer was given a dozen and the last one constituted the baker's profit.
According to the 1811 Dictionary of the Vulgar Tongue, by Captain Grose, "a Baker's Dozen is Thirteen; that number of rolls being allowed to the purchaser of a dozen".
Contrary to most sources, according to the 1785 Classical Dictionary of the Vulgar Tongue, Anon: probably Grose, "a Baker's Dozen is Fourteen, that number of rolls being allowed to the purchaser of a dozen".
The broadest use of baker's dozen today is simply a group of thirteen objects (often baked goods).
The term has also been defined in a jocular way, as "twelve of today's and one of yesterday's."
(Wikipedia)