Where does “-o-mno-” come from?

-o-mno- (Proto-Indo-European) comes from Proto-Indo-European -o-meno- — adverbial suffix of neuter gender.

Ancestry of “-o-mno-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	-o-meno-	adverbial suffix of neuter gender

-o-mno- — Etymology, Origin & Word History | Etymology Explorer