阿哥 is a colloquial term for 'elder brother' used in some Chinese dialects (especially in the south, like Cantonese and Hakka) and in historical or literary contexts. It can also refer to a male friend or acquaintance in a familiar, respectful way, similar to 'big bro' in English. In modern Mandarin, 哥哥 (gēge) is the standard term, so 阿哥 carries a regional or old-fashioned flavor.