to dwell: to make one's home in a place, or to reside within a person as a settled, active presence
The verb is built on the ordinary word for a house (οἶκος), and its plain sense follows from it: to have one's home somewhere, to live in a place, to settle there. When Paul addresses a believer married to an unbeliever, this is the everyday word for a couple sharing a household and staying together under one roof.
Its most striking use is for something that takes up residence inside a person. Sin "dwells in" Paul, and the Spirit of God "dwells in" the believer. The picture is not a brief visit but a settled, working presence, an occupant who lives in a house and is at work there. That is why the word carries so much freight in Romans, where the real question is who is at home in a person and running the place.
Greek has sharper compounds for the same idea: the related κατοικέω, "to settle down," stresses permanent residence, and ἐνοικέω, "to dwell within," spells out the inward sense. The plain verb here already stretches from a couple sharing a home to the Spirit making a home in a person, letting one word carry both the outward dwelling and the inward one.