I am pretty sure that the top comment is correct, it does mean prostitute in cockney rhyming slang but I read somewhere that it comes from people saying brass flute for prostitute which makes more sense to me then brass nail.
LIEUTENANT : "The operation was a success!"
COLONEL : "I noticed. I'm gonna bring it up with the Brass later on. Hope they accept the results though....you know how nervous and picky the Brass can be."