It is the dregs, the last of the wine, soda crackers, pickles, whatever is left. ”Bottom of the barrel” is not the barrel of a gun. I suppose that popped into my head recalling the two times I’ve had a gun pointed at me. But that’s another story.
The list grows shorter and I know you have been thoroughly invested in my efforts and idioms. Happily, I shall now allow my appliances to do some work – baking bread and washing clothes. They need a little boost in morale.