alienus

Alienus, aliena, alienum is first and second declension, built on alius. The core sense is "belonging to somebody else", and everything else runs out from it: foreign, strange, unsuitable, out of place, and at full stretch hostile. With an ablative, or with a and the ablative, it means removed from or out of keeping with something.

The phrase to learn is "aes alienum", literally another man's bronze, which is the ordinary Latin for debt. That single idiom explains why a Roman speech about aes alienum is about money owed and not about imported metalwork. Its natural opposite is proprius, one's own. English took alien, alienate and the legal alienation of property straight through, keeping the ownership sense the Romans had.