A term used mostly in Fanfic circles for a story idea. The term comes from the notion that the idea will constantly gnaw at your brain until you write it, or from the fact that these story ideas tend to breed like rabbits. Maybe both.
(Yeah, that's the ticket, rabbits that gnaw while they breed! It could be a kind of horror thing... or maybe a romance... the tragic story of "Elmerus, the Bunny Who Could Not Gnaw", doomed to a life of loneliness, ridiculed by the lady bunnies...)
Inspiration may come from Shipping, Crack Pairings (taking pleasure in just making it plausible), or any Ship Tease or Ho Yay left lying around. Even random phrases can spawn a plotbunny. For instance, Lewis Carroll wrote the nonsense poem The Hunting of the Snark backwards from the line "For the Snark was a Boojum, you see.", which popped into his head. He started wondering what a Snark was and why it shouldn't have been a Boojum, and who was seeking it...
Thus is fiction formed.