When somebody is unexpectedly quite physically strong. Everyone knew of the kids in highschool with down syndrome, which were incorrectly referred to as 'retarded'. Many of these kids are physically stronger than would be expected. Most of us have witnessed an event where one of these kids got in a phyiscal altercation with someone (or someTHING), whether based out of an argument or just horsing around, and the kid was surprisingly powerful.. probably stronger than anyone else at the school.
This is known as being Retard-strong.
(Term also found in the movie Clerks2, during the car ride scene back from the go-kart track)
Wow, that Billy is Retard strong!