Comes from the last names of "Kevin Stoley" and "Clyde Donovan", which are characters from the show South Park. Basically puts them together as a couple. Obvious evidence of this pairing can be found in the episode "Fatbeard" (Season 13).
One of the best fanfics ever for this pairing is "The Adventures of Tacoman and Roboinferno". You should go read it.
Like, RIGHT NOW. D:
Stolovan is one of the BEST pairings in South Park!