It's an archaic word for older brother. It originated as an honorific term of older brother, kinda like 兄上 (あにうえ； in Transformers Car Robot Speedbreaker addresses Mach Alert and Wild Ride as 兄上, likewise Mach Alert addresses Wild Ride as 兄上). These days it's used as a slang reference for any older brother figure and is commonly used in 'street slang' (including yakuza slang).

A lot of slang words in Modern Japanese come from archaic honorific terms. For example 貴様（きさま）is considered offensive today, but it was originally an honorific second person address (similar to 貴方(あなた); it even uses the same first Kanji); ditto お前（おまえ; although it's often slurred as おめぇ in gutter street talk).

I really don't recommend you start using these words in your 毎日会話 ... unless you want to sound like a chav. I don't recommend it though.

うっす！

P.S.: 兄上 is pretty safe if you want to use that. Although thou speech shalt verily forsooth soundeth of yon Oldendays. Asgaaaaaaaaard! *spins Mjölnir*