This is an informal expression, it means a person who bears a strong physical resemblance to another. Also called: spit, spit and image. This idiom alludes to the earlier use of the noun spit for “likeness”, in turn probably derived from an old proverb, “as like as one as if he had been spit out of his mouth” (c.1400). There are, however, other theories. Some of there are that spitting image comes from the words spirit and image. Another theory is that the expression drifted from splitting image, in witch a thing is split in two, thus providing an exact likeness.