"cunning as a sh*thouse rat"
No, I mean "exceedingly". I'm surprised you've never heard that simile (not metaphor) before, it's quite common where I come from, which happens to be where Nathan Greenham comes from as well.
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By that do you mean - not very?
I haven't heard of that one before, I heard a lot of "scrap c**t" or "nah jokes" on the coast, but not that.