Ironclad is the term for an unpopular pairing that is constantly bashed by the majority of the fandom. The only thing keeping the ship afloat is a good number of brave supporters. This pairing might or might not have some evidence, but it could be disliked for any number of reasons. These reasons might include Squick or it might be an awkward No Yay sort of ship.
Compare with Die For Our Ship, in [...] [tvtropes.org]