Technically, it means a person has a particularly low IQ, somewhere in the 60 range, if memory serves. My younger son, Noah, was recently subjected to IQ testing (part of his every-three-year testing regimen), and came out with a score of 48. I suppose that should make me feel awful, desperate, deflated. After all, he's officially mentally retarded, based on that score. But what I felt instead was that if in fact Noah's score represents him in any real way, then my sincere hope is that the world finds itself populated by people with IQs of 48.
After all, with more 48 Noahs in the world, we'd have a new population of people who are kind, who have no agenda, who aren't seeking to gain advantage over others, who only want to please people and make them happy, who have no urge to make money, to make war, to make other people feel bad. In fact, we'd have a whole new group of people who are entirely about being happy and making other people happy. We'd experience the simplicity of loving the people we're with, of having extraordinary radar for people who are genuine in their affection and concern, and we'd experience childhood wonder nearly every moment of every day.
My thanks go out as always, to Noah, for reminding me that those markers so many of us hold dear--educational attainment, job status, bank balances, etc.--can't hold a candle to what a "retarded" child like mine can give to the world. It's a whole new window into who really has ability in this world, and what it actually means to be disabled...