Where does “herissyttää” come from?

herissyttää (Ingrian) comes from Ingrian -ua, from Finnish -ttää, from Proto-Finnic -ttadak, from Proto-Uralic -kta-.

Ancestry of “herissyttää”, step by step

Step	Language	Word	Meaning
1	Ingrian	-ua	Used to form inchoative verbs
2	Finnish	-ttää	Front vowel variant of -ttaa
3	Proto-Finnic	-ttadak	Creates causative verbs from regular verbs,...
4	Proto-Uralic	-kta-	—