The theory is that the phrase "spit and image" referred to the image of a person reflected in a small pool of saliva, such as the one formed when someone spits on the ground. It emphasized how closely the reflection mirrored the person's true appearance.
While the phrase "spit and image" itself is rarely used today, the evolution to "spitting image" suggests that the phrase has lost its literal meaning and become more metaphorical. It now simply means someone who looks exactly like another person.
It's interesting to note that the phrase "spitting image" doesn't appear in writing until the late 19th century. This suggests that the phrase might have originated in colloquial speech and later gained widespread use.
Therefore, although the exact origin remains uncertain, the connection to reflections in saliva likely inspired the phrase "spitting image."