An inhabitant or permanent resident of a room or group of rooms below the ground level and usually under a building often a relative or child turned adult that refuses to leave the parents home.
After seeing the empty pantry, refrigerator and the recent electric bill Dad turned to Mom and said, " This is outrageous, tell cellar-dweller he needs to get a job and pay room and board we cannot afford him anymore."
One who labors in the cellar (or barrel room) of a winery never seeing the light of day. Quite like a rat.
Dude 1: What you doin' for the summer?
Dude 2: Ah, I'm a cellar rat at Eola Hills Wine Cellars.
Dude 1: Sick! That place has the bestSunday Brunch in Oregon!