Where does “kiihtää” come from?

kiihtää (Finnish) comes from Finnish kiihtyä, from Finnish -tyä — Front vowel variant of -tua.

Ancestry of “kiihtää”, step by step

Step	Language	Word	Meaning
1	Finnish	kiihtyä	to get excited; to become agitated; ~ = to...
2	Finnish	-tyä	Front vowel variant of -tua