Retard Units is an unofficial synonym for "imperial measurements". As the rest of the world use metrics, the internet culture started mocking the medieval traditions of the Americans, and how little sense the usage of imperials pose compared to metrics.
Space begins 100 kilometers from the earth, that's 62,14 miles in Retard Units.
The most vacuous, stone-brained way to say "totally retarded" - totes retar the word is in fact, totes retar.
Annoying White Chick 1: Oh gawd Sheila, the way Amanda wore pink and green together to prom was totes retar.
Annoying White Chick 2: Totally, gross, totes retar.