The OED etymology (Third Edition, 2008) suggest that the word is probably a Middle English development of Anglo-Norman segrestein (Latin sacristanus), which in turn was borrowed back into Latin (sextanus) and Anglo-Norman (sextain).
s.
1eccl.occupationsexton, sacristan, officer who is responsible for the maintenance of a parish church and its contents:
(
1311
)
La ou homme graunt eglise par assent de patron il y ad sextoun fet de moytez le patron presentera a resoun moyté
Ed II xi 65
(
1379
)
[...] et voille qe lez .ij. serges avantditz soient donez, aprés la servyse fyneez, al sextayn dez ditz freres
4
(
1428
)
jeo lise a priour et sextain de mesme le lieu [...] .c. li.
ii 382