To some people, the r-word is more offensive than the swear words referring to genitalia, sex acts or excrement. So, watch your mouth, you ******* re-... ...uh, -diculous ass!

Once upon a time “mental retardation” was a polite way of describing someone with mental developmental limitations - Somewhat less obnoxious at the time than “idiot, moron, or cretin” which were once medical terms. Eventually that pseudo-medical use of the term became corrupted in common usage and used as an intentional insult until now the term “retarded” or “retard” is perceived as an insult on par with the n-word or the b-word. Unfortunately your use of “retarded” probably hit Mom at two levels - (1) Maybe she believes her child (OP) is not that bright and she’s sensitive about that (It is essentially what he started off describing). (2) Now virtually any use of the r-word has become socially unacceptable and she’s sensitive about that too. It seems that no matter what word is used to describe someone towards whom part of society holds a negative image, eventually the word itself often becomes an unbearable insult and there are language taboos that build up against the word.

YDI for using that slur. Grow up.