Hoi is another form of bro, braddah, sistah, friend, etc.
Sah: that's such a hoi thing to do
Matt: wuts good
James: nothing much
Matt: ok boi
This is used as a greeting.
It is said the term comes from when Craig accidentally pressed "O" on msn whilst greeting a contact (Hi but pressed H,O,I)
The word was then used from that day on as it was cool.
This word has also been adapted by Dai to Hooooooooooooooooi
The longer you hold the pronunciation of it the greater you happiness is supposedly.
Tan: Aight son?
Craig: Gosh you're happy today