Where does “-ium” come from?

-ium (English) comes from Latin -um, from Proto-Italic *-om, from Proto-Indo-European *-oHom.

-ium (English): 1997, Bryan Pfaffenberger, "Official Microsoft...

Definitions

Step	Language	Word	Meaning
1	Latin	-um	nominative neuter singular of -us; accusative...
2	Proto-Italic	*-om	—
3	Proto-Indo-European	*-oHom	—