euphemism treadmill
Definition - A process of language change where a euphemism that has been coined to allow speakers to avoid saying a disreputable or unpleasant term itself becomes so associated with the disreputable or unpleasant thing that a new euphemism is coined to replace it - and so on and so on.

Example -
crippledhandicappeddisabledphysically challengeddifferently abled

Etymology -
According to the Wikipedia, the term was coined by Steven Pinker.

Note: I decided to follow-up on this, so I emailed Dr. Pinker and asked him if it was true that he had coined the term. He replied: "As far as I know I did, in a New York Times op-ed in 1994:"

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