Retarded has indeed taken on a second meaning – you even use it that way! You don’t literally mean that someone who overspends on tea, or doesn’t chap that they could buy a coffee and spill it out, is mentally retarded; you are disparaging their (or your) lack of astuteness.
It’s primary meaning is someone with limited IQ, it’s secondary meaning is an insult.
I thought you meant something else, does that make me retarded? 🙂