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a baker's dozen
Thirteen; one more than a usual dozen (12). When Jacob went to the bakery to buy doughnuts for the office, he made sure to get a baker's dozen so he could sneak one to eat on the way to work.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Thirteen, as in The new bagel store always gives you a baker's dozen. The origins of this term are disputed. One theory is that in times when bread was sold by weight, bakers who short-weighted their customers were heavily fined, and for safety's sake they would sell thirteen loaves for the price of twelve. Another theory is that dealers purchasing bread from bakers were allowed by law to receive thirteen loaves for the price of twelve, the thirteenth representing their cut of profit. [Late 1500s]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Thirteen. The source of this term is a law passed by the English Parliament in 1266, which specified exactly how much a loaf of bread should weigh and imposed a heavy penalty for short weight. To protect themselves, bakers would give their customers thirteen loaves instead of twelve, and in the sixteenth century this came to be called “a baker’s dozen.”
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer