Where does “asiakkuus” come from?

asiakkuus (Finnish) comes from Finnish asiakas, from Finnish asia, from Proto-Finnic acja — matter, case.

asiakkuus (Finnish): customership

Definitions

Step	Language	Word	Meaning
1	Finnish	asiakas	customer, client; client
2	Finnish	asia	issue, matter, affair, concern, business,...
3	Proto-Finnic	acja	matter, case

asiakkuus — Etymology, Origin & Word History | Etymology Explorer