Back Water Planets is a pejorative term that depending on the context and individual who uses it can describe different planets. There is usually an implication that the natives of those planets are primitives that have trouble understanding many elements of modern society.
While it's not uncommon to hear that term to refer to societies within the Union who have recently joined with a low Tech Level, there are some individuals who consider all but a few planets as "backwater planets."
 It is a common, somewhat arrogant expression known to be used especially on core worlds such as Pluribus Unum ( Pluribans consider pretty much anyone else to be of a Back Water Planet) Also of course Blue Moon, Sol Hub and other Core Worlds .
Natives to the worlds of SaresPrime,Natives to the Thauran Kingdom and traditional Sarans are also known to use this term to describe the heritage of anyone other than their own.