C16: from the bakers' former practice of giving thirteen rolls where twelve were requested,to protect themselves against accusations of giving light weight
thirteen in British English
(ˈθɜːˈtiːn)
noun
1.
the cardinal number that is the sum of ten and three and is a prime number
See also number (sense 1)
2.
a numeral, 13, XIII, etc, representing this number
3.
the amount or quantity that is three more than ten; baker's dozen
4.
something represented by, representing, or consisting of 13 units
determiner
5.
a.
amounting to thirteen
thirteen buses
b.
(as pronoun)
thirteen of them fell
baker's dozen in American English
thirteen
supposedly from bakers' former practice of adding an extra roll to each dozen soldto avoid any risk of penalty for short measure