Where does “filius” come from?

filius (Latin) comes from Old Latin fīlios, from Proto-Indo-European dʰeh₁y-li-os, from Proto-Indo-European dʰeh₁(y)- — to suckle, nurse.

Ancestry of “filius”, step by step

Step	Language	Word	Meaning
1	Old Latin	fīlios	—
2	Proto-Indo-European	dʰeh₁y-li-os	sucker
3	Proto-Indo-European	dʰeh₁(y)-	to suckle, nurse

filius — Etymology, Origin & Word History | Etymology Explorer