Emmylou Harris, Alison Kraus, and Gillian Welch wrote most of the lyrics to the version on O' Brother Where Art Thou. Sidney Hemphill Carter recorded a much simpler version in 1959 - in fact, it was just the first verse repeated over and over. I believe both versions are based on a very simple old Southern folk song / lullaby, which probably doesn't have much meaning to it. I think, and this is just my opinion, that the modern version is about slaves having to leave a young child unattended while they worked, which was pretty common. Maybe the mama died and the daddy is trying to raise the child without her. He has to take breaks to run home and give the baby a bottle, rock the child, etc, so the sugar doesn't get stocked. Some of the verses seam to be telling different stories.