I thought it was "a plan so cunning you could put a tail on it and call it a weasel."
:) I love that quote.

Speaking of cunning BlackAdder quotes;

Blackadder: Baldrick, I have a very, very, very cunning plan.

Baldrick: Is it as cunning as a fox what used to be Professor of Cunning at Oxford University but has moved on and is now working for the U.N. at the High Commission of International Cunning Planning.

Blackadder: Yes it is.

Baldrick: Hmm... that's cunning.

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And this has nothing to do with cunning, but it is from the same show as the above quote, and I find it rather funny:

"May I present to you, the greatest breakthrough in travel since Sir Rodney Tricycle thought to himself, "I'm bored of walking. I think I'll invent something with three wheels and a bell, and name it after myself": the time machine. "