From Diablo Wiki
In the Diablo community, "Ninja" is a term used to refer to stealing or sneaking something.
Just the word alone can mean to steal, as in, "He totally ninja'ed my item drop." The first-come-first-serve style of item drops from monsters in Diablo 2 was often called "ninja looting"
Ninja is often as an adjective with other words. For example, "ninja photos" or "ninja gameplay footage" would refer to surreptitiously recorded video footage of the game, such as might be recorded on a Blizzcon demo machine.