Someone who is your significant other, but whom it would be an insult to call "a boyfriend" (because it implies temporariness). "Boi" also implies a very deep bond of loyalty, almost like in a gang. A lot of goths and spooky kids use this word, often with "muh" or "tha" or "thee". I'm a little too old for that though.
Girls also use it to describe a good-looking goth or spooky kid.
That's my boi right here, and I love him to death.
If you act like that, my boi will fuck you up.
Mmmm...what a sexy goth boi! He looks like Reznor.
I'm supriused at how negatively this word is defined. In the "hardcore" culture, this word doesn't have such a negative connotation. Since "hardcore" girls are hard to get, if one is actually flirting with you and giving you some heavily sexual innuendos but being all coy and "unsure" about it, it means she is a "cock tease", which actuallhy means that she is seriously considering sex with you...
-I was reading my Anthropology textbook, which talked about rapists in fraternties using this word in a negative sense, and I was so surprised because I'd never heard it being used in that sense.
-The boi says I'm a cock tease but that's a good thing because that means he shall get what he wants ;-)