Where does “-isō” come from?

-isō (Proto-Indo-European) comes from Proto-Indo-European -yōs, from Proto-Indo-European -yoss.

Ancestry of “-isō”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	-yōs	Forms adjectives from roots, meaning "very" or...
2	Proto-Indo-European	-yoss	—