baker's dozen 的词源

Baker's dozen "thirteen" is from 1590s, said to be from an old custom.

These dealers [hucksters] ... on purchasing their bread from the bakers, were privileged by law to receive thirteen batches for twelve, and this would seem to have been the extent of their profits. Hence the expression, still in use, "A baker's dozen." [H.T. Riley, "Liber Albus," 1859]

这些商贩 [小贩] ... 从面包师那里购买面包时，法律允许他们以十二批的价格获得十三批，这似乎是他们利润的范围。因此，仍在使用的表达是，“面包师的十一个。”

But Brewer says the custom originated when there were heavy penalties for short weight, bakers giving the extra bread to secure themselves.

也比较一下 poulter's measure (an old verse style of 12- and 14-syllable lines), so called "from the varying number of a nominal 'dozen' of eggs" [Saintsbury, "History of English Prosody," 1906].*

* He adds: "Hot cross buns, I think, have (in worthy cases) preserved latest the generous fourteen to the dozen. Thirteen was pretty common and this, I believe, holds, against the author and in favour of the retailer, in the case of books."