What does "spitting image" mean?

A precise resemblance, especially in closely related persons. For example, Dirk is the spitting image of his grandfather. 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). The current idiom dates from about 1900.