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I'm seeing a lot of people using it in reference to Being Human
You might try asking at english.stackexchange.com/ or reddit.com/r/etymology but I'm not sure they can answer any more decisively than you could. If it is from Buffy, it might be mentioned in one of the articles or books about "Buffy Speak", "Buffy Slang" or "Slayer Slang".
How about "An American Werewolf in London" (1981)? Or do you include turning into a wolfman? Then that would be "Werewolf of London" (1935).
I think they meant the actual term "wolfing out," Konczewski, not just the general mythology of werewolves— obviously that's super duper old and predates Buffy by a lot! (Unless I'm totally off-base and they actually do use the phrase "wolfing out" in something from the 1930s, haha.)
An odd request: anyone know the first time 'wolfing out' (or to wolf out or to be wolfed out) was used? As in werewolves transforming into wolf form. If there's a better place to find out on this forum (or a better site to ask), let me know that, too.
...Was it Buffy? A lot of these are from Buffy. It sounds likely, now that I think of it. But I wanna know for sure.
Edited by Unsung