You can keep venomous in a rack - though I would strongly advise against it. Many people do it though. I just prefer to be able to ensure they are locked and I can see them anytime I do an enclosure check. Some states require you use locked enclosures - and do not consider racks secure enough to meet their requirements.
hot just refers to it being a venomous snake. Not entirely sure of the origin, though I'd imagine it comes from the description of a bite since it "feels like a hot knife being stabbed into your body"