Soul, ill-defined therefore unfalsifiable

(Originally posted by Jason in 8/2012)

There is a big campaign going on campus reaching out to student, courtesy of Harvest Mission Community Church. Since I happened to wear my most blatant "Michigan Atheist" shirt today (8/28), I was invited to join some of their students for dinner at Chipotle. Inevitably, familiar topics poped up, and one of my new friend asked, "Has soul been disproven?"
On the spot, I decided to go on asking why he believes in soul, why he believes that he has ONE soul instead of two, three, or multiple souls specialized in certain tasks (It's not as ridiculous as it sounds. According to the ancient Chinese belief, souls are composed of hun and Po, with 三魂七魄 "three hun and seven po" as one prominent dogma). And of course, the answer can only be the Christian Bible, which was written by extremely uninformed authors by modern standard, as I pointed out.
A little fun in conversation aside, those of us who are familiar with scientific method know that the problem is falsifiability: nothing observable can disprove the existence of soul. But at the bottom of it, what makes such concept unfalsifiable?
It seems to me that factual unfalsifiable claim is usually ill-defined, and vice versa. What's the details of soul? What's it composed of? Where and when was your soul created/made? Did the common ancestor of human and chimp have soul? If not, from which generation onward did we start to have soul? None of these questions can be answered according to the Christian faith. When I asked whether H. neanderthalensis, H. erectus, or H. habilis have soul, all they can answer is "if they are human". Unfortunately, ill-definedness is contagious: now the term "human" is ill-defined.
The same applies to claims like "God exists" and "There is a fire-breathing dragon in my garage", as neither god nor dragon is well-defined here. As people try to fill out the details or connect reality to unfalsifiable claim, it either becomes demonstrably false or imaginary/subjective: as long as it has nothing to do with reality, one can dream up all kind of things. In the end, "please elaborate" may be a more effective approach, compared to the invocation of fancy scientific method.