"spitting image" is used to refer to a person or thing which bears a very close resemblance
to something else. For example; "Bobby is the spitting image of his father"
The etymology of this phrase is unclear but there are two, connected, theories:
- The first theory suggests that the origin is the phrase "spit and image". The use of the word "spit" in this context has been seen in the phrase "as alike as if one had been spit out of the others mouth", (from 15th Century Britain).
- The second theory suggests that the phrase started life as "spirit and image", i.e. conveying the idea of a likeness that goes beyond outward appearance. "Spirit" turned into "spit" over time, to give "spit and image", (see above) which eventually became "spitting image".
Reference: Oxford Dictionary of English Proverbs