Where does “intrattenerci” come from?

intrattenerci (Italian) comes from Italian intrattenere, from Italian intra-, from Latin intra, from Latin interus, from Proto-Indo-European h₁énteros, from Proto-Indo-European -teros — Contrastive or oppositional adjectival suffix.

Ancestry of “intrattenerci”, step by step

Step	Language	Word	Meaning
1	Italian	intrattenere	to entertain; to establish or maintain; to...
2	Italian	intra-	intra
3	Latin	intra	within; inside; during; less than
4	Latin	interus	—
5	Proto-Indo-European	h₁énteros	inside, within
6	Proto-Indo-European	-teros	Contrastive or oppositional adjectival suffix

Every word from Proto-Indo-European -teros →Every word from Proto-Indo-European h₁énteros →Every word from Latin interus →