Someone that exhibits all the signs of mental retardation yet has somehow flown under the radar to date. A person eligible for the Special Olympics yet unaware of their eligibility, even if family and friends around the person see it clearly. A person of limited logic and decision making capabilities that is sometimes able to pass for normal.
Person B: Ralphy quit his management job yesterday because he thinks he can make more money bussing tables
Person A: Ralph is undiagnosed retarded... what did you expect?
Person B: True...