Since you seem to be asked this question frequently in a common situation, I would recommend coming up with one, "good," answer. Something creative and in-line with your personality (funny, sarcastic, dry, etc., as fits you) will give you good results much of the time.
Many of the people who ask you this are doing so in an effort to make small talk. Small talk is not question-and-answer, so direct, complete, factual answers to questions are often worse than vague statements as long as they leave large openings for continued conversation. Ideally, your canned answer would leave an opening like this. For anyone with whom you do not want to have a longer conversation, it is easy to end it later.
You have some reason that you don't want to talk about your origin; whatever reason you have is your business. Almost all of the people who ask you this don't know this reason and likely aren't trying to dig out uncomfortable facts. Reward their kind attempt at conversation with something that gives them opportunity to continue the conversation.
Just in case, for people who have less than kind intentions, a mostly direct but still slightly evasive answer will likely cause them to clarify their question (hence, "I mean originally"). They believe that they have an imperative to question you on an inappropriate conversational topic. This is similar to asking someone you don't know about the spirituality or religion they identify with or how much money they make. Likely, you will not have a good connection with most of these people anyway so the conversation is pointless.
A few direct examples, to choose based on your personality and adapt:
- Sarcastic/Dry: [pointing over shoulder] "Over there. Do I need to go back?"
- Brooding: "I've been trying to figure that out. Did you ever read [some recent armchair psychology book]?"
- Witty: "I'm from a tiny country wedged between Indonesia and Papua New Guinea. You should see how difficult it is for the flight attendants to figure out what meal to give you on flights from there."
Each shows that you've heard and understood the question and that you have intention of continuing conversation, which you may choose to end later. Yet each completely redirects the conversation from the subject you wish not to answer. If someone chooses to redirect back to your origin after one of these answers, they're clearly on a mission and you'll have to decide if you're willing to share cold, hard facts with them (based on their relationship to you).
As an indirect example that I use frequently, for when I'm asked, "how are you?" My answer is often, "living the dream." It matches my personality and it is frequently well out of line with my situation as the person whom I'm answering clearly observes. (For example, buying parts to fix a broken toilet.) I don't believe that most people asking the question truly want to know how I'm doing; they're either making small talk or being polite. If I get a quizzical look (e.g., from the cashier checking out my toilet parts), I often follow up with, "it's not my dream, but it's somebody's dream." More importantly, it is an opener for the person to continue conversation if they choose, with topics including (certainly not limited to, from experience):
- Living (being alive, happy, free, healthy, etc., which includes the weather)
- Dreams (or hopes, desires and aspirations), because I mentioned that
- Personal disposition (like, "you're very positive," or, "I'm not that positive")
- The situation at hand (like, "you dream of fixing toilets?")
- (Unsolicited) advice
- Diversion, possibly with the thought of, "that question didn't work, let me try another."
Most importantly with this example however, I do not want to answer this question because of my personal situation. Very rarely do I get a follow up forcing the question. In one case when it did happen, I was proactively told by a third party that the person forcing the question made the situation awkward by doing so.
Clearly, I don't do this with my doctor ("how are you doing?" is a necessary factual question) as you wouldn't with a customs inspector (for whom, "where are you from?" is a necessary factual question).