Not to pooh-pooh the Doobs and their smooth, aged take on swampy, countrified soul, but this show belongs to Steely Dan. (source)
I have checked several dictionaries but haven't found one listing a figurative usage of "swampy". As far as I know the word is only used to describe terrain. What does it mean in this sentence? What does it mean for soul music to be "swampy"?