Where does “-kac” come from?

-kac (Proto-Vietic) comes from Proto-Slavic -kati, from Proto-Slavic bьlkъ!.

Ancestry of “-kac”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-kati	Creates onomatopoeic verbs denoting an action of...
2	Proto-Slavic	bьlkъ!	—