IPA: /ˈruːmər/
KK: /ˈrumər/
A person who rents a room in someone else's house or property to live in.
The roomer paid his rent on time every month.
Roomer → The word is formed from "room" (from Old English *rum*, meaning space or area) and "-er" (meaning a person who). A roomer is a person who occupies a room, typically in a shared living situation.
Think of someone who occupies a 'space' ('room') — the '-er' just means a person. A roomer is someone who lives in a room.