Where does “fellatus” come from?

fellatus (Latin) comes from Latin fellō, from Proto-Indo-European dʰeh₁(y)- — to suckle, nurse.

Ancestry of “fellatus”, step by step

Step	Language	Word	Meaning
1	Latin	fellō	to suck, to suckle
2	Proto-Indo-European	dʰeh₁(y)-	to suckle, nurse