What will persist in our collective memory from last week's presidential debate, the second of three between John McCain and Barack Obama? The Philadelphia Inquirer suggests that only two remarks will be remembered: McCain referring to Obama as "that one," and Obama's defense against charges of naivete, "that somehow, you know, I'm green behind the ears." McCain's "that one" has already become an ironic catchphrase, even generating a website selling "That One '08" T-shirts. But what's the deal with "green behind the ears"? Didn't Obama mean "wet behind the ears"?

As you can see from the Visual Thesaurus wordmap, wet behind the ears is an idiom that means "lacking training or experience," synonymous with new and raw. And the wordmap for green shows two relevant senses: "not fully developed or mature" and "naive and easily deceived or tricked." So it looks like Obama uttered what some linguists call an "idiom blend" — combining two idiomatic expressions into a new fused version. Here are some more examples of idiom blends, which are sometimes created accidentally, and sometimes made with tongue firmly in cheek:

  • That's the way the cookie bounces.
    (blending "...cookie crumbles" and "...ball bounces")
  • It's not rocket surgery!
    (blending "rocket science" and "brain surgery")
  • He's really under the eight ball.
    (blending "under the gun" and "behind the eight ball")
  • That book is a real page-burner.
    (blending "page-turner" and "barn-burner")
  • Well, that's all water over the bridge.
    (blending "water under the bridge" and "water over the dam")

Green has long been used as the color of immaturity, playing on the metaphor of unripe fruits or plants. The Oxford English Dictionary has examples of green in the sense of "inexperienced" back to the mid-16th century. Shakespeare used it in a memorable play on words in Antony and Cleopatra: "My salad days, when I was green in judgment." A similar term is greenhorn, meaning "an awkward or inexperienced youth." Here the metaphor is from the animal kingdom, presumably from a name for young cattle with "green" or immature horns.

Animal husbandry is also the likely source for the other part of the idiom blend, wet (or not yet dry) behind the ears. Here's how Charles Earle Funk explained it in his 1948 classic A Hog On Ice:

A saying that came directly from the farm, where many others have also arisen, for it alludes to a newly born animal, as a colt or a calf, on which the last spot to become dry after birth is the little depression behind either ear. The figurative use seems to be wholly American, too homely to have attained literary pretensions, but undoubtedly in familiar use through the past hundred years or longer.

Though Funk is probably right that the expression is originally an Americanism, the earliest example I've found so far is actually from a British writer, the notorious Edward Bulwer-Lytton (1803-1873). Lord Lytton is now chiefly remembered for the line, "It was a dark and stormy night," which has become so identified with bad prose that it has inspired the Bulwer-Lytton Fiction Contest for "the funniest opening sentences from the worst novels never written." In The Parisians (a novel published in 1873, the year of his death), Lord Lytton puts these words in the mouth of an American character, Colonel Morley: "Sir, a man may go blind for one gal when he is not yet dry behind the ears, and then, when his eyes are skinned, go in for one better."

The "green behind the ears" variation is actually rather old. In the 1911 book The Compleat Oxford Man, A. Hamilton Gibbs writes, "When these people have the needle what a remarkable change comes over them. Some tremble and look green behind the ears." That's not quite the same meaning, it seems to me: it's playing with green in the sense of "looking pale and unhealthy," as in the expression "green around the gills," so it could be a different kind of idiom blend. But this example from 1924 looks like a predecessor of Obama's usage (and in the context of presidential politics, no less):

Of one thing all can be sure: that so long as the law stands as it does today Republican presidents will continue to appoint Republican postmasters and Democratic presidents will appoint Democratic postmasters. He is quite green behind the ears who either expects otherwise or that either party is going to change the law and deprive themselves of the opportunity afforded by it.
Charleston (W. Va.) Daily Mail, Sept. 9, 1924

More recently, "green behind the ears" has cropped up in numerous places. (Two British uses: it was the title of a 1984 book by Faith Addis describing her move from London to the countryside, and in 1999 the Scottish rock band Travis released a song by that name.) For some, it may simply be a jocular takeoff of the established idiom "wet behind the ears." Others might treat it more seriously: after all, the literal meaning of "wet behind the ears" only makes sense to those familiar with raising livestock, so why not replace it with "green behind the ears," which has the added advantage of evoking the "immature" associations of green?

You can hear Obama use the phrase at about 30 seconds into this clip. Judge for yourself whether the apparent idiom blend was intentional or accidental — and if it was intentional, whether it was serious or facetious. Perhaps he just wanted us to forget about another much-discussed livestock idiom, "lipstick on a pig"!