Where does “kasto” come from?

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

Ancestry of “kasto”, step by step

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

kasto — Etymology, Origin & Word History | Etymology Explorer