An unusual relationship of any kind of fandom (i.e. books, television shows, movies, ect.) that is unlikely to happen. In fact a person must be on crack to even support such a relationship.
Draco Malfoy and Frodo Baggins is the best crack ship.
Crack ship, crack!ship, or crack pairing is a term often used in fan fiction or role playing that pairs two people pairing from different fandom universes into a romantic relationship. This can be anything from a tv show, video game, or a book. A crack ship usually makes the readers question "Why such an off pairing" Some people might like it, some hate it, and others might be grossed out.
"There saw this crack ship up on this role playing site i was on and the person paired Chace Crawford with Kiki Kannibal."
"Ugh, that's really gross."