E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bakers Dozen.
Thirteen for twelve. When a heavy penalty was inflicted for short weight, bakers used to give a surplus number of loaves, called the inbread, to avoid all risk of incurring the fine. The 13th was the vantage loaf.
1
Mr. Riley (Liber Albus) tells us that the 13th loaf was the extent of the profit allowed to retail dealers, and therefore the vantage loaf means, the loaf allowed for profit.
2
To give one a bakers dozen, in slang phraseology, is to give him a sound drubbingi.e. all he deserves and one stroke more.