A persons behind which appears similiar in size and shape to a bucket?, maybe a vessel full of cr..? A huge behind, good for nothing except sitting on. Maybe a reference that someone is lazy? Not sure of background but an Aussie male I know used it all the time even to people without large behinds.
"What have you been doing all day? bucketarse!
A man who loves having enormous numbers of oversized penis' being pounded continuously and relentlessly into his anus'; thus stretching it and making it loose and leathery, often dripping discharge and other body fluids, mostly male sperm and puree';d faeces.
"that Matty in the Admiralty Towers in Brisbane, Australia, is a dirty, loose legged, slug bucket-arse faggot, ay?