Where does “sub-aqueous” come from?

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

Ancestry of “sub-aqueous”, step by step

Step	Language	Word	Meaning
1	English	aqueous	Consisting mostly of water
2	English	-ous	Used to form adjectives from nouns, to denote...
3	Middle English	-ous	—
4	Old French	-us	-ous, used for forming adjectives
5	Latin	-ōsus	ose, -ous, -ful (full of or overly prone to)
6	Old Latin	-ōssus	—

Every word from Old Latin -ōssus →