[wao]

I'm sure there's a more detailed etymology of that term, but afaik it literally means what it is: neta (ネタ) meaning something like "content", "stuff", "material" and bare (バレ) meaning "exposed". Put them together and you have "content exposed" = spoiler.

I'd like to know when it first started being used and so on though. I don't think it's a particularly anime-confined term either... I think.