According to urban dictionary:
An official declaration made by patrons of an establishment who feel they have been cheated. Once a charge of shenanigans has been accepted by an authority figure, said patrons are free to assault the owners of said establishment with brooms.
"Man, this WoW GM busted my ass for duping items, and then I catch him hooking one of his friends up with free gold. I'm calling shenanigans!"
Also see, shenaniganization:
When a normal and/or serious coversation or message board thread becomes overrun and distorted by silliness and general tomfoolery.
"You are witnessing the shenaniganization of another thread."