1) Used by trade guilds for 'craft' or 'trade'.
The York cutlers described their craft as a ‘mystery’, and an order in 1477 allowed eny maister apprentez or servant to wirke in the said misterie by candel light under certain conditions. In 1565, the ordinances of Sheffield’s cutlers spoke of the cutler occupacion and cuttelers craft whereas that had changed in 1590 to the mystery or crafte of a Cutler and the said science or crafte. This was ‘mystery’ as the word was used in the medieval trade guilds, derived from Latin ministerium, and meaning ‘craft’ or ‘trade’: the noun ‘mister’ was in use from the thirteenth century, comparable with French ‘métier’. There was almost certainly some confusion with ‘master’ and ‘mastery’, and also with ‘mystery’ in the sense of secret ceremonies which had an identical spelling but different origin. That may be implicit in article 4 of the Sheffield Orders which employs the plural of the word, referring to personnes usinge … the said mysteryes or scyence. If the term ‘little mesters’ has a long enough history it may have been influenced by that confusion.