Where does “lassukkain” come from?

lassukkain (Ingrian) comes from Finnish -kkäin, from Finnish -käs, from Proto-Finnic -k'as, from Proto-Finnic -s, from Proto-Finno-Ugric -s.

Ancestry of “lassukkain”, step by step

Step	Language	Word	Meaning
1	Finnish	-kkäin	Front vowel variant of -kkain
2	Finnish	-käs	Front vowel variant of -kas
3	Proto-Finnic	-k'as	Forms adjectives from nouns, indicating a...
4	Proto-Finnic	-s	Forms nouns
5	Proto-Finno-Ugric	-s	—