Where does “แข้ง” come from?

แข้ง (Thai) comes from Proto-Tai ɣeːŋᴮ, from Middle Chinese 脛.

แข้ง (Thai): shin

Definitions

Step	Language	Word	Meaning
1	Proto-Tai	ɣeːŋᴮ	shin; lower leg
2	Middle Chinese	脛	—

แข้ง — Etymology, Origin & Word History | Etymology Explorer