Hi, Mrrzy. My dictionary says that spit can mean 'exact likeness or image,' and merely notes that it is dialect. No origin. Spitting image used to be spit and image, as the OED observes.
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Here's a word I'm tired of: passion. So many people nowadays seem to be looking for their passion, blah blah blah. I have a lot of interests and causes, but I don't believe I've ever had a passion for anything.

What would I have to do if I had a passion for something? Stay up night and day doing it? Spend all my money on it? Roam the world seeking it? I have better ways to spend my time.