There's a related, more polite version of "are you retarded" which is not uncommon even here on HN. It is "I'm confused". I don't know whether it's a phrase that I'm over analysing, but it always comes across as disingenuous to me.
The responder is never actually confused, they have a question that they should just ask.
The responder is never actually confused, they have a question that they should just ask.