Where does “-nka-i” come from?

-nka-i (Finnish) comes from Finnish -i, from Proto-Uralic -j — oblique plural.

Ancestry of “-nka-i”, step by step

Step	Language	Word	Meaning
1	Finnish	-i	A nominal suffix used in eg. ukko → ukki; Used...
2	Proto-Uralic	-j	oblique plural