If someone calls you “the spitting image” of your mother, you probably understand them to mean that you look exactly like your mother. But, like up to snuff, made from scratch, and countless other phrases, the literal meaning of spitting image is pretty cryptic to most modern-day English speakers.
In fact, it’s a little cryptic to linguistic scholars, too: There’s not a single accepted explanation for how spitting image came to be. As Merriam-Webster reports, the leading theory is that the phrase started out as spit and image—spit having been used to mean “exact likeness” since at least the early 19th century. How spit got that connotation is another mystery, though some believe it’s a less wordy way of saying “You look so much like so-and-so that you must’ve been spit right out of their mouth!” That sentence showed up almost exactly in works dating from the late 16th century and onward, including in George Farquhar’s 1698 play Love and a Bottle. “Poor Child! He's as like his own Dadda, as if he were spit out of his mouth,” Mrs. Trudge declares.
Eventually, you could simply call someone “the very spit” of someone else and have your meaning understood. In an 1810 entry from The New Newgate Calendar, for example, a nurse described a child as “the very spit of the old Captain.” From there, the phrase apparently evolved to spit and image, which may have been misheard enough to become spitten image, which gave way to spittin’ image, spitting image, and even splitting image.
As for whether spitting image is a mistake depends on how long you think a phrase needs to be around in order for it to be considered technically correct. According to the Oxford English Dictionary, the earliest known written reference to spitting image was in 1929—and spittin’ image first appeared back in 1901.
But as previously mentioned, not everyone subscribes to this linguistic trajectory for the expression. Some scholars think it’s a truncation of spirit and image, another (albeit much less common) way to highlight similarities between two people; or that English speakers co-opted it from a French phrase. It’s also been suggested that spit doesn’t refer to saliva at all. As Yale University linguistics professor Laurence Horn wrote in a 2004 paper later cited in the Chicago Tribune, spit may be an allusion to “a rather different bodily fluid … inherently more relevant to the transmission of genetic material.”