baker's dozen

Noun
1.
baker's dozen - the cardinal number that is the sum of twelve and one
13, long dozen, thirteen, XIII
large integer - an integer equal to or greater than ten

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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]