the bilge is the part of a ship below the floor boards of the bottom level, just above the hull... ie the darkest and most grim part of the boat. a rat is, well, a small annoying (and to some people, scary) rodent. Therefore the term "bilge rat" is a pirate insult.
arrr, ye be a great foolish bilge rat, ye be!
- Entry for "Bilge Rat" on Urban Dictionary