You've probably heard a dozen theories on the origins of "spitting image." You probably haven't heard this one.

Our lesson in wacky idioms began when reader Steve Cianci wrote us to lament the use of the term, maintaining that "spit and image" is the proper phrase. "The very spit of someone is an exact likeness," he wrote. "'The spit and image' or 'spit image' emphasizes the exactness."

Like so many colloquialisms — beat the band, piece of cake, knock on wood — spitting image has been around for hundreds of years, so it's impossible to nail down its exact origins. But we turned to Laurence Horn, professor of linguistics at Yale University, who is widely held as an expert — if not the last word — on the topic.

Cianci is in good company with his "spit and image" theory. "For the majority of language columnists, spittin' image is a euphemistic alteration or 'corruption' of the original expression, spit and image," Horn wrote in a paper on the topic.

Others maintain the true phrase is "splitting image," as though a person had been split into two mirror images.

Still others, Horn says, insist the saying is a melding of "spirit and image," as in "he's the very spirit and image of his father," meaning he's got his dad's spirit and looks.

But none of those theories stands up to Horn's research.

"I argue, with support from history, dialects, and other languages with parallel expressions, that the original form was actually 'spitten image,'" he told us, before passing along the 25-page paper he wrote in 2004 for the linguistics journal American Speech called "Spitten Image: Etymythology and Fluid Dynamics."

("Etymythology," Horn explained, is an offshoot of etymology: "The invention of lexical urban legends that explain various expressions in the language without any actual historical support for them.")

The paper came about after he spoke to the annual meeting of the American Dialect Society on etymythology in general, and spitting image specifically. The editor of American Speech, who was in attendance, urged him to write up his presentation, and he's been fielding "spit" questions ever since.

"Spitten," Horn points out, "is the dialectal past participle of spit." A word you rarely see these days, but a word, nonetheless.

Over the years, "spitten" morphed to "spittin," which morphed, finally, to "spitting."

Here's where it gets interesting.

"Spitten image," he says, refers to "a likeness that was literally spit out, but where figuratively the 'spit' in question involved a rather different bodily fluid."

"It has further been shown," Horn wrote in his paper, "that the motivation of this participial form — attested in a wide variety of languages — rests in the analogy between spit and another bodily fluid … inherently more relevant to the transmission of genetic material."

Which explains why most historical uses refer to a father and son.

So is Horn known to launch into a protracted — and carefully substantiated — defense of "spitting image" when he hears someone say "spit and image" or "splitting image"?

"No," he says. "Like other linguists I never correct. I just observe and, when appropriate, record."

Wikileaks. Gleek. Refudiate. We've got a list of favorite words from 2010. What's on yours? E-mail Heidi at hstevens@tribune.com.