Basically, crack is silly, intentionally non-serious, and has a high tendency to be extremely crazy and unfounded, such as purposeful OOC
-ness or playing on cliches, inside jokes, and/or fanon
The term crack probably stems from crack as in crack cocaine, as opposed to a crack in a wall. As in, so horrendously unfounded/silly that only one could have come up with it while under the influence of crack.
Ed/Roze and Kagome/Kohaku are my secret crack pairings in the Fullmetal Alchemist and Inuyasha fandom
Kagome/Sake, Inuyasha/Tessaiga, Inuyasha/Kagome's Seiraa Fuku, Shinidamachuu/Kikyou, Naraku/Kikyou's Ashes, and Jakotsu/Hairpin are also crack pairings - these are all from Inuyasha as well.