Retarded is a word for slow, especially mentally slow. This is a medical word that spread to slang, and is today considered very insulting and inappropriate.
Some uses of retarded are harmless, like if you said wearing a backpack retarded or slowed down your progress on a hike. However, if you use retarded to mean stupid, you're on dangerous ground. Out of respect for people who have clinical mental retardation, this is considered an offensive term to throw around. But there are other terms that moved from the medical world to the slang world that are OK to use, including idiot and moron.
