What's hard for you to understand about the word "voluntary?"

A man points a gun at me. He says: Give me the money, or die. I make the voluntary decision to give him the money.

how the word "voluntary" is actually used by people is complex in interesting ways. People report (degrees and shades of) involuntariness and coercion in lots of different situations. For example "I hate this shitty job but I don't have a choice". So as a matter of language use there's lots of different kinds of involuntariness that doesn't cleanly map onto any libertarian-ish view. At which point libertarians tend to make the move of proposing their own, much more narrow definition of "voluntary". Which implicitly embeds normative assumptions that would need to be argued for, which libertarians fail to do.