Scottish: From Caithness dialect, some who is crazy, mad, stupid, idiotic, more of an exclamation than an insult, rather like saying he is "aff his heid!" A cull is someone who is not wise, similar to a radge or radgepacket, does daft things for a laugh! I think the bucket was just tacked on later because it sounds funnier!
Dinnae listen till at cullbucket, he is full o wind and pish!!