When you spend a good deal of time in the sand, whether it be at the beach or other sandy areas, and you come to realize that you have sand caked up in your lower groin. It creates a noticable discomfort known as "dune crotch."
Dune digga dune is used to describe someone who is overweight or fat. It is derived from the opening theme of the cartoon "Fat Albert" when the bass line first starts making the sound "dune digga dune digga dune digga dune" followed by the unforgettable "hey hey hey, its faaaat Albert".