It might be a Scottish name, taken from a story about two men in a train. One man says, "What's that package up there in the baggage rack?" And the other answers, "Oh that's a McGuffin." The first one asks "What's a McGuffin?" "Well," the other man says, "It's an apparatus for trapping lions in the Scottish Highlands." The first man says, "But there are no lions in the Scottish Highlands," and the other one answers "Well, then that's no McGuffin!" So you see, a McGuffin is nothing at all.
In the strictest interpretation, a MacGuffin is an object or item the pursuit of which drives the plot, but which has no real impact on the story. The maltese falcon in The Maltese Falcon is a classic MacGuffin: it serves solely to motivate the characters. Some cite the stolen “letters of transit” in Casablanca as another classic MacGuffin, but by the narrow definition of MacGuffin, they aren’t: the letters of transit allow two people to leave Casablanca, and two people leaving Casablanca is vital to how the whole movie evolves and reaches its climax.
The broader interpretation of MacGuffin allows the object to have more significance in the story. The MacGuffin in such cases may be laden with symbolism, or emotional significance to the characters, and may actually play a pivotal role in the course of the plot. George Lucas has claimed that R2-D2 is the MacGuffin of Star Wars. In the narrow sense, he’s wrong, as R2-D2 is a character whose actions profoundly influence the story; and the plans that are stored inside his memory are what allows the Rebel Alliance to destroy the Death Star. That’s hardly hardly having little real impact on the story other than acting as a motivator!
The narrow use of MacGuffin implies interchangeability. The maltese falcon could have been a jewel or a rare book or the blueprint for a rocket without altering the characters or the way the story unfolds; you could sit down and alter the novel or the screenplay without having to change the scenes or even much of the wording.
In the expanded use of MacGuffin, as used by Lucas, the MacGuffin is an integral part of the premise of the story, and if you change your MacGuffin you dramatically change the shape of the story. Change R2-D2 to a secret formula or a a jeweled crown, and you will find that Star Wars requires quite a rewrite—and may not, in fact, be workable as a story.
At the risk of being accused of objectifying women, I could argue (and in fact shall) that in the expanded sense of MacGuffin, Mary is the MacGuffin of There’s Something About Mary. (Come to think of it, I didn’t write the screenplay, and if there’s objectification going on it would be in the script, not in my comments here. So stop raising your eyebrows at me.) Mary is the motivator for everyone’s behavior, and the structure of her life and personality affect the course of the story—replace her with a very dissimilar love interest, and the story changes. Replace her with a money-filled suitcase, and the whole story vanishes.
Why am I talking about this? Because I’m having MacGuffin problems—MacGuffin being in the expanded sense of the term. (I rather doubt that I’ll ever write a story that employs a MacGuffin in the narrowest sense of the word.) And so I’m musing about it in public.
I suppose the plural is simply MacGuffins. But I prefer to think of them in Gollum-speak. How many MacGuffinses does your story have, precious?