'Boyyy' simply means nothing can ever beat it, there is no other word that can top 'Boyyy'. If something is Boyyy you know you couldn't miss it for the world.
me: oh man did you see that girl last night mate: No bro I didn't
me: Trust me she was BOYYY
Your mate does a triple summersault, that was BOYYY
It means whatever the fuck Bu wants it to mean at the present moment. Can also be followed by a 'to that'. Whenever the 'to that' is used, you are owned extremely bad.