Oh - yes, it's one of the meanings, but nowadays this word isn't really offensive (I haven't heard anyone using it like this). Word-for-word (and much nicer) meaning is "dog's brother" ("brat" means "brother" in Polish). It's a kinda pet name for people who like/care about dogs. Many animal rights' associations use this word (for example - this one => http://www.psubraty.pl/index.php?option=com_content&task=view&id=26&Itemid=61)