Where does “ຢູ່ໃນ” come from?

ຢູ່ໃນ (Lao) comes from Lao ຢູ່, from Proto-Tai ˀjuːᴮ — to be.

ຢູ່ໃນ (Lao): in, within

Definitions

Step	Language	Word	Meaning
1	Lao	ຢູ່	to be; to live, reside, abide; to live, exist
2	Proto-Tai	ˀjuːᴮ	to be

ຢູ່ໃນ — Etymology, Origin & Word History | Etymology Explorer