[maxerickson]

Clinton was first called Slick Willy by journalists who thought he misrepresented his politics on the campaign trail:

As best as I can determine, thanks to the help of the Pine Bluff public library, it had it's origin on September 27, 1980, shortly after Bill Clinton gave a speech before the state Democratic convention in which he depicted himself as in the tradition of progressive governors in this state, an assertion that offended us at the Pine Bluff Commercial because we thought of him as more of a trimmer who had broken this succession of reform governors, from Winthrop Rockefeller, to Dale Bumpers, to David Prior. And so we used the sobriquet, Slick Willy on that occasion and it caught on.

https://www.pbs.org/wgbh/pages/frontline/shows/choice/bill/g...

I'm sure others had their own reasons for using it.