Where does “diminutiivisuus” come from?

diminutiivisuus (Finnish) comes from Finnish -uus, from Proto-Finnic -uc — -ness, -hood.

Ancestry of “diminutiivisuus”, step by step

Step	Language	Word	Meaning
1	Finnish	-uus	Forms nouns from adjectives, expressing a quality
2	Proto-Finnic	-uc	-ness, -hood