Have you ever had to prove a point? If so, you probably needed to find evidence that could not be denied — that was absolutely true. That evidence would be considered irrefutable, impossible to disprove.
The adjective irrefutable comes from the Latin verb refutāre, "repel or beat," with the added negative prefix in-. Some things are simply irrefutable. These are things based in logic (if a=b and b=c, then a=c), or in fact (George Washington was the first president of the United States). But some things that are irrefutable, while maybe not logical, are simply not arguable, as when someone responds to "Why not?" with "Because I don't want to"!