Used to describe someone who is not fully retarded (as in someone who cannot successfully use the restroom without supervision), but is retarded enough to cause them to make extremely bad judgment calls. Usually a semi-retard is capable of regular activities but often decides to do something really, incredibly stupid that either injures them or makes them a spectacle.
O.J. is semi-retarded for thinking that "What happens in Vegas stays in Vegas" after his crime.
Bryson: (Mad because he got cut by a breaking bit when trying to drill through a concrete block)
You: What the hell Bob?! Are you semi-retarded?