Where does “-ous” come from?

-ous (Middle English) comes from Old French -us, from Latin -ōsus, from Old Latin -ōssus.

Ancestry of “-ous”, step by step

Step	Language	Word	Meaning
1	Old French	-us	-ous, used for forming adjectives
2	Latin	-ōsus	ose, -ous, -ful (full of or overly prone to)
3	Old Latin	-ōssus	—