from bartleby
spit and image, spitten image, spittin’ image, spitting image, very spit of


The key to this puzzling cliché is the word spit, which seems to have been an early-nineteenth-century dialectal noun meaning “an exact likeness or counterpart of a person or thing,” as in He’s the very spit of his father. The question is, how did spit develop that sense, dialectal or not? The Oxford English Dictionary concludes that it developed somehow from the noun spit, meaning “expectorated saliva,” but no really satisfactory explanation of the semantic change involved has thus far turned up. In the late nineteenth century the full cliché developed: He’s the very spit and image of his father, followed by the folk etymology that replaced spit and with spitting (or spittin’ or spitten) image. The phrase has a rather countrified air about it, and it is not likely to be found today in either Formal writing or at the higher levels of speech
not that illuminating so i tried
http://babel.uoregon.edu/romance/rl407/borrowing/borrowing.html
this threw a bit more light on it but http://www.takeourword.com/Issue019.htmlwas better, word detective had this to say on the matter (scroll down to the end)http://www.word-detective.com/052598.htmland the linguist hotly debated it some years agohttp://www.emich.edu/~linguist/issues/4/4-780.html, well thats all well and good but do i have anything to say of my own? not really, 'cept, i liked the telly programme when i was a kid