In the good old days, Kodak used to publish a set of books called Dataguides that had all kinds of useful information. The darkroom dataguides included a "standard" negative and print; the idea was that the user had a practical reference of what a well exposed, well-developed negative and print looked like. He could also use this negative as a standard test to calibrate his own equipment or fine-tune his procedures. (Later Dataguides did not include these, probably as a cost-saving measure. A shame.)
These standard images featured a pretty caucasian female model; since the model was not identified, somewhere/somehow she acquired the nickname "Shirley," and the negatives (or transparancies) were referred to as Shirleys.
I have seen a couple of explanations as to how the name came to be: one is that it was the name of one of Kodak's preferred models. But I've seen nothing to support it and I consider it apocryphal.
Another person told me that it came from "surely".
I think it's far more likely that someone just came up with the name, it stuck, and spread by word of mouth over the years. They could have just as easily been Bettys, Peggys, or Lisas.
if you're an English major, you could make the case that it might come from Herman Wouk's1955 novel, Marjorie Morningstar. In it, there's a discussion of how many women turn into Shirleys, a middle-class, suburban wife-and-mother with all the desires and trappings that lifestyle entails. Even though Marjorie Morningstar was a best-seller and Natalie Wood starred in the movie, the term never entered into popular culture the way, say, "security blanket" did. On the other hand, "Shirley" didn't have to be in the minds of the public at large; it only needed to be in the minds of one or two influential people who used the term to describe the anonymous, "average" women in these photos.
Back to the topic at hand: according to Bill C. on photo.net, these came from printer control negatives, which would be used by labs to set up their printers. I believe there would be a Shirley for each film stock (example #1 was shot on Ektacolor), but how many were actually available is anybody's guess.
So there are various and sundry Shirleys out there. How many? I have no idea. So I've created this web page to collect as many as I can find.
Do you have a Shirley that's not here? Please scan it and email it to me and I'll include her!
There's a relevant discussion of Shirleys on this thread at photo.net
There's a story from NPR's radio show Morning Edition: "How Kodak's Shirley Cards Set Photography's Skin-Tone Standard"