As we all know, proverbs and figures of speech give language a colorful idiomatic depth. Sometimes their origins are completely lost in the fog of changing customs and attitudes; other times, batty folk explanations and urban legends arise (analogous to folk etymologies). Such is apparently the case with “rule of thumb”, for instance.
So, as much as we enjoy a clever or colorful bon mot in the real world, it can be even more fun to come across one adapted for use in a fictive world. An example from Tolkien that pops readily to mind is Bilbo’s worry over “[e]scaping goblins to be caught by wolves!” We’re told by the narrator of The Hobbit that “it became a proverb, though we now say ‘out of the frying-pan into the fire’ in the same sort of uncomfortable situations.” 
Reading the Harry Potter books again, I’ve just come across some examples of this in The Order of the Pheonix that I though I’d share. They’re all uttered by a flustered Mrs. Figg at the outset of the story. First comes, “we might as well be hanged for a dragon as an egg” . This one is particularly nice because it calls to mind the episode with Hagrid and the Norwegian Ridgeback, Norbert, in The Sorcerer’s Stone. Dragons are banned by the Ministry of Magic, of course, so having even an egg is already breaking the law; probably some kind of magical misdemeanor. The closest Muggle equivalent of the proverb would probably be, “in for a penny, in for a pound.” That is to say, if you’ve gone in part of the way, you may as well go all the way.
The next two follow soon after. “[I]t’s no good crying over spilled potion,” and “the cat’s among the pixies now” . Okay, “spilled potion” is a bit lame, I admit, and its Muggle counterpart is only too obvious. But I like “the cat’s among the pixies.” We know all about pixies from Gilderoy Lockhart’s risible antics in The Chamber of Secrets; they’re annoying, but basically harmless. But a cat would have a field day chasing them — one pictures something like Crookshanks chasing the Weasleys’ garden gnomes at the Burrow, only with a lot more chaos. This is probably the wizarding wording of “a bull in a china shop,” eh?
The trick in all this is knowing how far to go so that the adaptation of proverbs for a fictive world doesn’t become ridiculous — or perhaps I should say, riddikulus. Otherwise, where does it all end? “If the Sorting Hat fits, wear it?” Or maybe: “That’s putting the cart before the thestral?” Or even: “You curse your mother with that mouth?!” :)
But salting the dialogue in a story with a few cleverly adapted proverbs can spice it up nicely, don’t you think? Can anyone think of other amusing or clever mock-proverbs from fantasy and science fiction? I have a feeling Gaiman must be chock full of them, though nothing comes immediately to mind.
 Tolkien, J.R.R. The Annotated Hobbit. Ed. Douglas Anderson. Boston: Houghton Mifflin, 2002, p. 145. Anderson also provides a footnote on the origins of the real proverb being subsumed into Tolkien’s fictive world.
 Rowling, J.K. Harry Potter and the Order of the Phoenix. New York: Scholastic, 2003, p. 21
 Ibid., p. 24.