Where does “într-un” come from?

într-un (Romanian) comes from Romanian întru, from Latin intrō, from Latin intra, from Latin interus, from Proto-Indo-European h₁énteros, from Proto-Indo-European -teros — Contrastive or oppositional adjectival suffix.

într-un (Romanian): with masculine and neuter nouns: in a; into a

Definitions

with masculine and neuter nouns: in a; into a

Ancestry of “într-un”, step by step

într-un traces back through two separate lines of ancestry.

via Romanian întru

Step	Language	Word	Meaning
1	Romanian	întru	in
2	Latin	intrō	to enter, go into, come in, get in, penetrate
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

via Romanian un

Step	Language	Word	Meaning
1	Romanian	un	a, an
2	Romanian	preună	—

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