The East End of London, (UK), has long been famous for it's Cockney rhyming slang, "Apples & Pears" = stairs, "Dog & Bone" = 'phone, "Ginger Beer" = queer(gay), etc. so "Septic Tank" = yank.
Not very polite but probably not quite as insulting as it may first appear!
A whole lot more about it
here! .