Where does “萬里長城” come from?

萬里長城 (Chinese) comes from Chinese 萬, from Proto-Sino-Tibetan s-maŋ.

Ancestry of “萬里長城”, step by step

Step	Language	Word	Meaning
1	Chinese	萬	10,000
2	Proto-Sino-Tibetan	s-maŋ	—