There are offensive terms for pretty much everything and there are idiots in this world that like to harass good people. Try not to let them bug you for if they know they bug you, satisfaction is instantly granted. If they did this at work, you could try saying it bothers you, ask the perpetrator politely to stop, or failing that, tell management you are not happy with the verbal abuse you receive and it makes you feel uneasy. The word "retard" is daft anyhow.