Old English bæcere "baker, one who bakes (especially bread)," agent noun from bacan "to bake" (see bake (v.)). Cognate with Dutch bakker, German Bäcker, Becker. In the Middle Ages, the craft had two divisions, braun-bakeres and whit-bakeres.
White bakers shall bake no hors brede..broune bakers shall bake whete brede as it comyth grounde fro the mylle withoute ony bultyng of the same. Also the seid broune bakers shall bake hors brede of clene benys and pesyn, And also brede that is called housholdersbrede. [Letterbook in the City of London Records Office, Guildhall, 1441]
Baker's dozen "thirteen" is from 1590s.
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]
But Brewer says the custom originated when there were heavy penalties for short weight, bakers giving the extra bread to secure themselves.
Baker, to spell, an expression for attempting anything difficult. In old spelling-books, baker was the first word of two syllables, and when a child came to it, he thought he had a hard task before him. [Barrère and Leland, "A Dictionary of Slang, Jargon & Cant," 1897].