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 best Sunday Brunch in Oregon!
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.