as sensible as a dictionary
In Lewis Carroll's Through the Looking Glass and what Alice found there, in the chapter The Garden of Live Flowers, the Red Queen is talking to Alice about what she's been up to:
"I only wanted to see what the garden was like, your Majesty -"
"That's right," said the Queen, patting her on the head, which Alice didn't like at all, "though, when you say "garden" - I've seen gardens, compared with which this would be a wilderness."
Alice didn't dare argue the point, but went on: "- and I thought I'd try and find my way to the top of that hill -"
"When you say "hill"", the Queen interrupted, "I could show you hills, in comparison with which you'd call that a valley."
"No, I shouldn't," said Alice, surprised into contradicting her at last: "a hill can't be a valley, you know. That would be nonsense -"
The Red Queen shook her head. "You may call it "nonsense" if you like," she said, "but I've heard nonsense, compared with which that would be as sensible as a dictionary!"
Alice curtseyed again, as she was a little afraid from the Queen's tone that she was a little offended: and they walked on in silence till they got to the top of the little hill.
Thanks to Simon James for the text and to Sean Gugler for the URLs.