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.
The name buckethead is commonly used between fisherman to describe annoying destructive usally eurpoean descent fisherman. The name buckethead comes from the fact they sit on their buckets all day and fish. These fishermen pollute the enviorment, drink, and keep all the fish they catch.