Spitting image is the longstanding modern formulation of the idiom meaning exact likeness, duplicate, or counterpart. The original phrase was spit and image, inspired by the Biblical God‘s use of spit and mud to create Adam in his image. But spitting image has been far more common than spit and image for over a century.
A few writers still use spit and image, but trying to keep the original idiom would be a lost cause. Though spit and image is the original, it’s also distracting to readers who have been hearing spitting image their whole lives. Of course, spitting image can be just as distracting to some careful readers. Perhaps the phrase is best avoided entirely.
Examples
Web searches show around 50 instances of spitting image for every spit and image. Most major publications have accepted the change—for example:
But Olsen is delighted with her beloved horse’s spitting image. [Washington Post]
American folk singer Dani Shay is a spitting image of the teenage heartthrob … [Telegraph]
It doesn’t hurt that at just 12 years old, Bianca is already 1.86m tall and the spitting image of her mother. [New Zealand Herald]
Suffice it to say that Stalin’s son, the spitting image of his father, strides onto the scene … [Wall Street Journal]

