No version of the phrase is especially old. The earliest reference is in Andrew Knapp and W. Baldwin's The Newgate Calendar, 1824–26: "A daughter, ... the very spit of the old captain."
And we get to the first known use of 'spitting image' - in A. H. Rice's Mrs. Wiggs, 1901: "He's jes' like his pa - the very spittin' image of him!" (source)
It's meaning is "exact likeness." Here are some spitting images I found...that make me giggle.