Where does “-āgium” come from?

-āgium (Latin) comes from Old French -age, from Old Occitan -atge, from Latin -āticum.

Ancestry of “-āgium”, step by step

Step	Language	Word	Meaning
1	Old French	-age	forms nouns with the sense of 'action or result...
2	Old Occitan	-atge	suffix used to form masculine nouns
3	Latin	-āticum	—