The word 'brother'
 

I have recently taken to calling a friend of mine aniki (because I have heard this term used in many animes, and seen it used online), because he is very much like a brother to me.

But I know, like everyone does, that the internet can be wrong. So tell me, what does aniki mean? Who uses it, and to whom?

兄貴 (Aniki) means big brother. 弟 (Otouto) means younger brother.

Are there specific instances where one would use it, or with a certain age group?

I am only asking this because a friend has disputed me about this, telling me that oniichan (excuse my spelling) means brother, not aniki.

Aniki can mean older brother, but usually means a superior of some sort (so it's kinda like honoring your older brother).

Oniichan is more affectionate.

Edit: An example is the Yakuza uses Aniki, I think.

Just call him XXXちゃん. That's what guy long-time friends call each other if they're particularly close. I have a couple Japanese friends I do this with.

The Japanese guys I know love talking to me because I joke around and call them XXX-sama. They find it hilarious (it's a gag, of course).

I would use Aniki only if he is your actual brother, he is your mafioso superior, or he did something incredible for you, like save you from drowning or pulled you from a fire, and you owe him a "life debt". And of course he must be older than you, too.

Exactly, going back to the whole 'superior' thing. Aniki is a bit... eh.

I had a friend named Fumi in Japan, and we were teasing each other back and forth during a big tennis club event, and I called him ふみやろう. The team loved it and for some reason that nickname stuck. I think maybe we were punning on the word やろう a week earlier, and so the name was one of those postmodern "it's funny because you're referencing our shared history" type deals.

"it's funny because you're referencing our shared history" is a long way to say 'running gag', hahaha.