A word used by the stereotypical chav in either an online conversation or via text message as an alternative to the word 'boy'.
There is no REAL reason why they use the word 'boii', but the two i's (ii) seem to be pretty popular among the chavs in the 14-18 age group.
There are other words such as
iino (i know)
Whats worse, is that some people use the word 'boi', Message to the people who say 'boi', this doesnt make you one of those lazy typers, as its the SAME amount of words as 'boy', it just makes you look stupid.
Grace: rarrtii dat boii woz buff
Meaning: That boy was attractive