1.
big brother; older brother
An informal, somewhat masculine way to refer to one's older brother. More casual than 兄 or お兄さん.
2.
bro; boss; senior (as address)
Used as a term of address or reference for a respected older male figure, particularly in yakuza contexts, street culture, or among close male friends. Implies camaraderie and respect.
A masculine, informal word. Women more typically use お兄ちゃん to refer to an older brother.
In yakuza or gang contexts, 兄貴 is a standard term for a senior member. In everyday usage, it can express affectionate respect for an older male friend or mentor.
The female counterpart is 姐さん (older sister figure), commonly used in the same contexts.