Where does “pääaine” come from?

pääaine (Finnish) comes from Finnish pää-, from Finnish pää, from Proto-Finnic pää, from Proto-Uralic päŋe — head.

pääaine (Finnish): major

Definitions

Step	Language	Word	Meaning
1	Finnish	pää-	main, head, prime, primary, principal, leading
2	Finnish	pää	head; end, tip
3	Proto-Finnic	pää	head
4	Proto-Uralic	päŋe	head

pääaine — Etymology, Origin & Word History | Etymology Explorer