If you can put "just" before "lie" then you're not taking this seriously, you've never found yourself really, badly lied to, or you have a serious dearth of ethics.
Don't lie. Deflect, even if it's bluntly. Saying "I don't believe that's relevant, but I will tell you how much I'd like to make" pretty much says "I'm underpaid" but also says "I know I'm underpaid and I expect better," which tells them something useful about you that you actually want them to know.
Call it what you want; lying to a prospective employer means you no longer have any grounds to expect them to behave in an honest way. If you can't be honest with people you have no reason to expect honesty in return.
If my current company discussed my salary with anyone outside of my company or myself, there would be much larger problems. No reasonable company would be dumb enough to disclose personal information about one of their employees.
Don't lie. Deflect, even if it's bluntly. Saying "I don't believe that's relevant, but I will tell you how much I'd like to make" pretty much says "I'm underpaid" but also says "I know I'm underpaid and I expect better," which tells them something useful about you that you actually want them to know.