Originally a derogatory Greek term, now appropriated to apply to anyone behaving like a nob, tool or wanker. Especially relevant to a group of nobs, tools or wankers. The collective noun for a group of boostis is a 'mangle' of boostis.
The term has been further used as derogatory slang between close friends.
Warren: What a boosti. You gonna say something?
Darren: Nah... look at his mates... what a fucken mangle of boostis... all roided up and wearing ed hardy t-shirts...
Warren: I'm going to the bar
Darren: Get us a beer will ya, boosti?