Used to describe a person without street smarts; i.e. lacking the common sense required to deal with the potential dangers of real lifeoutside of school.
1: Sure, he gets straight A's and all, but he's extremely streetarded.
2: Damn.
Your everyday normal person, who is not medically retarded, but acts in a way that makes you think, "man, that dumbfuck is retarded!" Used as to not offend medically retarded people.