Why Is The Word "Retard" Considered Offensive When The Word Emerged To Replace Earlier Labels?

True retard was used before but that was in the 'medical' model of disability in which people with disability were branded as being pathetic and unable to contribute to society.

In most developed countries we have the social and human rights models of disability who advocate for the empowerment of people with disability. Words such as retard are both offensive (like you said) because of the history of the medical model but also because of the social reference since lots of people associate 'retarded' with stupid, idiot, etc so it's only natural that its deemed offensive.

Also many people with disabilities are trying to redefine their own terms, so its not the deaf but Deaf, since their trying to empower themselves.

Bottom line: just please don't use it!

Some meaning of words changes with time, if I could remember, GAY used to be : happy, light-hearted or brightly colored etc but now we don't use "gay" to describe happiness anymore, b'coz thru' time gay takes more meaning/usage to label someone's sexuality.

Word "retard" has the same predicament, b'coz of frequent usage it changes in meaning and it's now offensive to use it on someone. We have to keep ourselves contemporary and not be caught in this dilemma with choice of words.

Because, if it ever really was used by medical professionals or others as a precise non-judgmental term, they were far outnumbered by school kids who used it as an insult.

Languages are not fixed in stone, they are ever evolving creatures, so you can't apply what was true 1 century ago to today. I guess the word became offensive the moment it started to be used offensively by regular people like you and I, for instance if someone started offending you by saying, 'you are such a retard' or 'you are so retarded' I'd have a hard time not considering this an insult. Even the word 'ignorant' itself was first used to indicated one's disinterest (to ignore, in the original Latin sense, meaning 'not caring'), then it came to be used as an insult as well.

It's considered offensive for the exact same reason the "N word", once just a word like any other, is considered offensive; because people began using it in an offensive manner.

Nothing in your discussion is wrong in any way. It's just that it doesn't matter what the history of the word is. What matters is where history has taken the word today.