1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): a baker’s dozen
thirteen. (Bakers often added an extra item to an order for a dozen.) • We ended up with a baker’s dozen each of socks and undershirts on our shopping trip.
McGrawhill's American Idioms And Phrasal Verbs