“The term is used so broadly in contemporary conversation that usage is no longer based primarily on assumptions about specific behaviors of people who have mental retardation – just the general assumption that retardation is bad, something to be avoided, and things, ideas or people described as retarded should be excluded from the attention of non-retarded people. At this point, the connotation is simply “that’s bad and you should ignore it.” (See the Urban Dictionary entry for the term, which describes it as meaning “bad” in literally hundreds of different ways.) And that is ableist – using a word that not only describes but is the actual medical diagnosis of a mental disability to mean “bad and ignorable.” Using the term reinforces the implicit assumption that mental disabilities are bad and that people with mental disabilities should be excluded and ignored because of their disabilities. And that affects all people with mental disabilities, not just those diagnosed with mental retardation or another developmental disability. (Although it is especially difficult for family members of people with developmental disabilities.)”
Ableist Word Profile: “retarded” - DisabledFeminists
This is a long and polite way of saying “stop saying it because it’s really fucked up and shitty when you do”.