Where does “几” come from?

几 (Chinese) comes from Proto-Sino-Tibetan krəj — foot, leg; footstool, seat, chair.

Ancestry of “几”, step by step

Step	Language	Word	Meaning
1	Proto-Sino-Tibetan	krəj	foot, leg; footstool, seat, chair

几 — Etymology, Origin & Word History | Etymology Explorer