Something untouched has literally not been touched (like the untouched spinach on your plate), or has not been emotionally moved (like the hardened criminal who remains untouched throughout his victim's testimony).
If you're not moved to some emotion, you're untouched, and if your dog doesn't eat his dinner, you'll see an untouched bowl of kibble still sitting there in the morning. You might find it hard to remain untouched by the fresh, white, untouched snow. This adjective comes from touched, which can mean "emotionally moved" or "physically affected." Add the "not" prefix un-, and you get untouched.