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The only reason words like "moron" are no longer deemed grossly offensive is precisely because people started using those words to describe things other than their literal meanings and over time their impact as slurs and insults has been reduced, and this is exactly what has happened with the word retarded. For probably the majority of the English speaking world the word has already reached this watered down status. Languages evolve, meanings change, sometimes rapidly, any attempt to regulate it will fail.