Used to describe anything so convenient at the current time that it defeats the purpose of trying to figure out why it just happened to be there.
As the burglar neared ever closer, Amy grabbed the oh-so-convenient letter opener that for some reason was taped against the wall she was backed up on. Then, she stabbed wildly at him, and he fell to the floor.
With a grin stretching across her face, she drags the body to the well in her backyard (wtf?!) and pushes him in. It was then that we realize that that was no burgler —— that was another of her victims.