Never heard the term.
Never heard the term.
Why don't you tell us?
I'm usually very careful about Urban Dictionary, but it's pretty much accurate on this phrase. If you look through several of the definitions given there, it hits most of the cultural aspects of the term, more or less:
Urban Dictionary: Retard strength
Just like an intelligent person can be a physical retard, a mental retard can be a natural athlete. And when it happens, it can be amusing.
Excited delirium. Usually some cocktail of responses to threat stimuli, medications or disease.
People get strong quick. If you've ever watch videos of multiple officers wrestling one skinny meth head. Or heard of soccer moms deadlifting cars.
Sourced from people with mental retardation not understanding or being aware how much strength to use when dealing with others in various social conditions (hugs/play/etc.).
Same as old man strength.