genetic drifter
Person conceived of breeding between two closely related couples, or someone with a very weak gene pool, but usually the latter. Places famous for being rather inbred are some remote or scummy parts of the UK (where the chav population is high) and isolated US redneck areas. Used as a polite way of saying "inbred" non-perjoratively, or in an ironic manner.
I've tried explaining why people need electricity, but those genetic drifters don't seem to understand! It's frustrating!