Where does “plagiarius” come from?

plagiarius (Latin) comes from Latin plagium, from Ancient Greek πλάγιος, from Ancient Greek -ῐος, from Proto-Indo-European -yós — Creates adjectives from noun or verb stems.

Ancestry of “plagiarius”, step by step

Step	Language	Word	Meaning
1	Latin	plagium	manstealing, kidnapping, the selling of freemen...
2	Ancient Greek	πλάγιος	placed sideways, slanting, aslant
3	Ancient Greek	-ῐος	belonging to; of”, “belonging to”, “pertaining to
4	Proto-Indo-European	-yós	Creates adjectives from noun or verb stems

Every word from Proto-Indo-European -yós →

plagiarius — Etymology, Origin & Word History | Etymology Explorer