Where does “kopu” come from?

kopu (Finnish) comes from Finnish -u, from Proto-Finnic -u — Forms nouns from adjectives, nouns.

Ancestry of “kopu”, step by step

Step	Language	Word	Meaning
1	Finnish	-u	Forms nouns from verbs. Most common with "e"- and...
2	Proto-Finnic	-u	Forms nouns from adjectives, nouns