Where does “吾” come from?

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

Ancestry of “吾”, step by step

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