According to this article, the first mention of the phrase "social distancing" occurs in 2006 in The New York Times:
If the avian flu goes pandemic while Tamiflu and vaccines are still in short supply, experts say, the only protection most Americans will have is “social distancing,” which is the new politically correct way of saying “quarantine.”
The H5N1 ("avian flu") outbreak in 2006 prompted the Bush administration to develop a plan around social distancing, though they never implemented it.
The article cites a bunch of contemporaneous quotes from prominent people rejecting the idea on various grounds. I think this is selective quoting by the author, since the Bush administration endorsed social distancing:
The [Bush] administration ultimately sided with the proponents of social distancing and shutdowns ... Then the coronavirus came, and the plan was put to work across the country for the first time.
Bush bites us from beyond the (political) grave.