Someone who is not quite yet a bro, but almost certainly will be after just one more party or gathering of the bros. Also used for females, from greek demeter and demi, a feminine prefix, to denote a female who is close to the bros.
1) "That John is a real bro"
2) "Nah, I dont know him, he's more of a demibro"
"Man, I'm glad Emma's here, she's a real demibro"
A girl who, except for lacking a penis, would qualify as a bro. She is interested in drinking, fucking, partying, working out, and chilling with her bros.
Demi-bros can sometimes be problematic, on account of her wanting to have sex with her bros, and vice versa. This is the only thing that disqualifies her from being a bro, because bros should not want to have sex with their bros.
That girl is such a demi-bro, buy her another beer.