Septic tank = Yank
http://www.cockneyrhymingslang.co.uk/slang/septic_tank
I didn't even do it right, it should have been "We septics just don't have a clue." The advanced form just
suggests the rhyme so you really have to know what's going on.
Also, this is one way the Brits are better than us (and I do believe that they are), they can subtly insult you and you didn't even know it happened.