Where does “chaston” come from?

chaston (Old French) comes from Frankish kastō, from Proto-Germanic kastô — vessel”, “container.

Ancestry of “chaston”, step by step

Step	Language	Word	Meaning
1	Frankish	kastō	box, case; box”, “case; chest, reservoir
2	Proto-Germanic	kastô	vessel”, “container

chaston — Etymology, Origin & Word History | Etymology Explorer