Where does “ŋoː” come from?

ŋoː (Proto-Tani) comes from Proto-Sino-Tibetan ŋa, from Proto-Sino-Tibetan l/b-ŋa — five.

Ancestry of “ŋoː”, step by step

Step	Language	Word	Meaning
1	Proto-Sino-Tibetan	ŋa	I
2	Proto-Sino-Tibetan	l/b-ŋa	five